2622 [android_alarm_manager] remove MainActivity references  (cla: yes)

2624 [google_sign_in] Migrate to Android v2 embedder. (cla: yes)

2627 [google_maps_flutter] Rename ‘Page’ in the example app to avoid type conflict with the Flutter Framework. (cla: yes)

2628 [path_provider] Fix misaligned enum values in StorageDirectoryMapperTest (cla: yes)