Save generated previews in AppWidgetService

This change updates AppWidgetServiceImpl to persist generated
previews in /data/system_ce/<user>/appwidget/previews. Each file
contains the previews for a single provider, written as a
GeneratedPreviewsProto message.

Previews are cleared when a provider app is updated, deleted
(PACKAGE_REMOVED) or its data is cleared (PACKAGE_DATA_CLEARED).

Also updates the bug for the feature flag to the correct number.

Test: Manual, set previews and reboot, then clear data and remove
 package.
Test: AppWidgetTest#testGeneratedPreviewPersistence
Bug: 364420494
Flag: android.appwidget.flags.remote_views_proto

Change-Id: I7c800eeab84480675514e236cac06fe3deb43fac