Simplify communal widget loading

We introduce CommunalAppWidgetViewModel to move only the necessary IPCs
to a background thread, while keeping everything else on the main
thread. This also simplifies things by removing the container FrameLayout
we previously used to asynchronously load widgets.

Fixes: 382744855
Test: atest CommunalAppWidgetViewModelTest
Flag: EXEMPT bugfix
Change-Id: Ia2ae8a493467e7ca5d52e6994d4a282acb8ffef0