[quick_actions] Support v2 android embedder.

[cyanglaz]

Description

Minimum work to migrate to the android v2 embedding.

Related Issues

flutter/flutter#41845

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[bparrishMines]

LGTM

Still needs the snippet explained by flutter/flutter#42348

[bparrishMines]

Suggested change

	MethodCallHandlerImpl handler;
	private MethodChannel channel;
	private MethodCallHandlerImpl handler;

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[matthew-carroll]

Should "some unique action key" be written here?

[cyanglaz]

I don't have enough context to give input here. It seems this is introduced in 63609c9
and @collinjackson or @BugsBunnyBR might be able to give some inputs on this.
Anyways, this is not related to this migration. I suggest we create new issue and fix it in a later PR if needs to be fixed.

[juliocbcotta]

That is just a unique string, the value itself does not matter.

[cyanglaz]

@BugsBunnyBR We can make the value of the String unambiguous so it is more readable.

[matthew-carroll]

LGTM from a new plugin API perspective.

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