[espresso] Enable warnings as errors

[navaronbracke]

This PR enables javac lint options for espresso and fixes the violations.

List which issues are fixed by this PR. You must list at least one issue.
Part of flutter/flutter#91868

If you had to change anything in the flutter/tests repo, include a link to the migration guide as per the breaking change policy.
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[navaronbracke]

@stuartmorgan I managed to fix all violations, except one error (which is reported twice):

/Users/navaronbracke/.gradle/caches/transforms-3/eb8527de6e1dbbf4b7e85b2278bb4bc3/transformed/espresso-idling-resource-3.5.1-api.jar(/androidx/test/espresso/IdlingRegistry.class):
warning: [classfile] MethodParameters attribute introduced in version 52.0 class files is ignored in version 51.0 class files

Do you have a clue on how to fix this one? Per https://mvnrepository.com/artifact/androidx.test.espresso/espresso-idling-resource we have the latest version of that dependency.

[stuartmorgan-g]

Do you have a clue on how to fix this one?

From what I found, we can't; see https://github.com/flutter/packages/blob/main/packages/google_maps_flutter/google_maps_flutter_android/example/android/build.gradle#L38-L40

[CaseyHillers]

test-exempt: Lints were updated, and they're verified in presubmit tests

[reidbaker]

Open questions about annotation scope but overall this looks good.

[stuartmorgan-g]

This should be first, and have the **BREAKING CHANGE** annotation described at https://github.com/flutter/flutter/wiki/Contributing-to-Plugins-and-Packages#changelog-style

[navaronbracke]

Done

[stuartmorgan-g]

This one isn't a breaking change. It should be:

* **BREAKING CHANGE**: Migrates uses of the deprecated [...]
* Changes the severity [...]
* Aligns [...]

[stuartmorgan-g]

LGTM, thanks!