Add get/setWidgetPreview API in AppWidgetManager

This change adds an API to AppWidgetManager that allows providers
to store generated RemoteViews in AppWidgetService, where they
can be retrieved by hosts. Currently these previews are not
persisted between reboots.

Bug: 308041327
Test: adb shell device_config put app_widgets \
    android.appwidget.flags.generated_previews true
Test: CtsAppWidgetTestCases
Change-Id: Ib536b4ad20b75245780933dde18e4f14b4ee0ae3