[in_app_purchase_android] Add showInAppMessages support (#107412)

[fingerart]

As mentioned in #107412, a new native feature (showInAppMessages) has been added.

final androidPlatformAddition = InAppPurchase.instance.getPlatformAddition<InAppPurchaseAndroidPlatformAddition>();
final result = await androidPlatformAddition.showInAppMessages();

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[gemini-code-assist]

Code Review

This pull request adds support for the showInAppMessages API from the Google Play Billing Library. This includes defining the new API in Pigeon, implementing it on the Android native side, adding Dart wrappers, and updating the example app to demonstrate its usage. Tests for the new functionality have also been added.

[gemini-code-assist]

For better maintainability and to explicitly handle any future InAppMessageResponseCode values, it's good practice to include a default case in your switch statement. This makes the code's intent clearer and prevents new values from being silently treated as NO_ACTION_NEEDED.

Suggested change

	static @NonNull PlatformInAppMessageResponse fromInAppMessageResponseCode(
	@InAppMessageResult.InAppMessageResponseCode int inAppMessageResponseCode) {
	switch (inAppMessageResponseCode) {
	case InAppMessageResult.InAppMessageResponseCode.SUBSCRIPTION_STATUS_UPDATED:
	return PlatformInAppMessageResponse.SUBSCRIPTION_STATUS_UPDATED;
	case InAppMessageResult.InAppMessageResponseCode.NO_ACTION_NEEDED:
	return PlatformInAppMessageResponse.NO_ACTION_NEEDED;
	}
	return PlatformInAppMessageResponse.NO_ACTION_NEEDED;
	}
	static @NonNull PlatformInAppMessageResponse fromInAppMessageResponseCode(
	@InAppMessageResult.InAppMessageResponseCode int inAppMessageResponseCode) {
	switch (inAppMessageResponseCode) {
	case InAppMessageResult.InAppMessageResponseCode.SUBSCRIPTION_STATUS_UPDATED:
	return PlatformInAppMessageResponse.SUBSCRIPTION_STATUS_UPDATED;
	case InAppMessageResult.InAppMessageResponseCode.NO_ACTION_NEEDED:
	default:
	return PlatformInAppMessageResponse.NO_ACTION_NEEDED;
	}
	}

[fingerart]

@stuartmorgan-g @GaryQian What else do I need to do?

[stuartmorgan-g]

@fingerart You need to wait for a review, which as our docs explain takes up to a couple of weeks.