Allow App Widget Overlays by Product Configuration.

This changelist adds support for loading a set of app
widget overlays to be shown over dreams from config.
Products can specify the components to show along with
the position within the parent by gravity.

Bug: 201676043
Test: atest AppWidgetOverlayPrimerTest
Change-Id: Ic2f706598f776e0802106e333c4003242227faea