Enable plugin infra for both iOS and Android post-#9791.

Brings hello_world and flutter_gallery Android code in line with the
corresponding iOS code.

This reverts commit 8da4778e.

* Update hello_world to new gradle based build flow

* Update flutter_gallery to gradle based build

* Align hello_services with new gradle based build

* Update platform_services to new gradle based build

* Upgrade stock to gradle based build

* s/sky/flutter/ in Android templates

* update engine.version with a compatible engine version

* replace more SkyActivity references with FlutterActivity

This is needed to update an app's layout when a soft keyboard is shown

* making flutter run work with gradle

* locate android studio

* add test for settings

* review comments

Instead, require an AndroidManifest.xml and always build an APK.

Fixes #2517

This allows us to adjust exactly which gestures we're listening for
during layout, which I'll use to kill a SizeObserver.

@abarth @hixie

Very simple so far. This schedules an alarm to fire once a day, kicking
off a service that downloads a new app.skyx from a hardcoded URL. The
new skyx replaces the current one.

@abarth

@abarth

@abarth

This is just one of many steps in preparation for turning
fitness into a play-store ready app.

@abarth @collinjackson