[image_picker] Mention launchMode: singleInstance in README

[JeroenWeener]

Users of the image picker are reporting issues for Android when setting the launchMode of the application activity to singleInstance. Namely, the image picker will always return RESULT_CANCELED. After investigation, it turned out that this problem is inherent by the way Android handles activities with this launch mode: singleInstance instructs the framework to open new activities in a new task. As activities cannot communicate between tasks, the image picker activity is unable to relay back the result to the calling activity.

This PR updates the README to mention this behavior and provide a workaround.

Closes 76856.

Pre-launch Checklist

• I read the Contributor Guide and followed the process outlined there for submitting PRs.
• I read the Tree Hygiene wiki page, which explains my responsibilities.
• I read and followed the relevant style guides and ran the auto-formatter. (Unlike the flutter/flutter repo, the flutter/packages repo does use dart format.)
• I signed the CLA.
• The title of the PR starts with the name of the package surrounded by square brackets, e.g. [shared_preferences]
• I listed at least one issue that this PR fixes in the description above.
• I updated pubspec.yaml with an appropriate new version according to the pub versioning philosophy, or this PR is exempt from version changes.
• I updated CHANGELOG.md to add a description of the change, following repository CHANGELOG style.
• I updated/added relevant documentation (doc comments with ///).
• I added new tests to check the change I am making, or this PR is test-exempt.
• All existing and new tests are passing.

If you need help, consider asking for advice on the #hackers-new channel on Discord.

[tarrinneal]

just a couple formatting changes

[tarrinneal]

nit: break this up into multiple lines, similar to how we've formatted other blocks of text in this README.

[JeroenWeener]

How would you want me to break up the lines? The way it is currently done in the README does not strike me as being consistent.
Should I limit lines to the 80 character count? Or should a start a newline when it makes sense for the flow of the text?

Option 1:

**Note:** Launching the image picker from an `Activity` with
`launchMode: singleInstance` will always return `RESULT_CANCELED`. In this
launch mode, new activities are created in a separate [Task][2]. As activities
cannot communicate between tasks, the image picker activity cannot send back
its eventual result to the calling activity. To work around this problem,
consider using `launchMode: singleTask` instead.

Option 2:

**Note:** Launching the image picker from an `Activity` with `launchMode: singleInstance` will always return `RESULT_CANCELED`.
In this launch mode, new activities are created in a separate [Task][2]. As activities cannot communicate between tasks, the image picker activity cannot send back its eventual result to the calling activity.
To work around this problem, consider using `launchMode: singleTask` instead.

Combining options 1 and 2:

**Note:** Launching the image picker from an `Activity` with
`launchMode: singleInstance` will always return `RESULT_CANCELED`.
In this launch mode, new activities are created in a separate [Task][2].
As activities cannot communicate between tasks, the image picker activity
cannot send back its eventual result to the calling activity.
To work around this problem, consider using `launchMode: singleTask` instead.

[stuartmorgan-g]

We generally try to keep lines to 80 characters in the READMEs; there's no enforcement so it's not always done consistently. Either the first or third option is fine.

[tarrinneal]

I'd go with option 3 here.

If you feel like making the rest of the readme fit this rule as well, that would be appreciated! You don't have to though.

[tarrinneal]

Now that we have 2 Notes, maybe it would be a good idea to add a section for Critical Information or something like that, to avoid collecting multiple "tag" endings on this section.

[JeroenWeener]

Sounds good. Would that still fall under the Android heading? Example:

### Android

...

#### Critical information
Images and videos picked using the camera are saved to your application's local cache, and should therefore be expected to only be around temporarily.
If you require your picked image to be stored permanently, it is your responsibility to move it to a more permanent location.

Launching the image picker from an `Activity` with `launchMode: singleInstance` will always return `RESULT_CANCELED`.
In this launch mode, new activities are created in a separate [Task][2]. As activities cannot communicate between tasks, the image picker activity cannot send back its eventual result to the calling activity.
To work around this problem, consider using `launchMode: singleTask` instead.

[stuartmorgan-g]

Yes, a subheading of Android as you've shown there would definitely be the right approach.

[stuartmorgan-g]

In fact, even better might be to make each of these instead be a new #### subheading, and also move Handling MainActivity destruction on Android and Android Photo Picker up here as #### headings as well. Currently those are free-floating rather than under Android, even though they are Android-specific (and the first is a similar sort of note to this one).

@tarrinneal What do you think?

[tarrinneal]

I think @stuartmorgan has the right idea here.

[stuartmorgan-g]

The new text looks great! LGTM once Tarrin is happy with the structure.

[tarrinneal]

Thanks for doing the extra work with this one!