Move widget view inflation to the main thread

We currently create the app widget host view in a background thread
since it involves blocking IPCs. However, this is causing view inflation
to also happen in the background which is breaking some widgets. This
change ensures the listener callbacks are always executed on the main
thread, so ensure view operations happen on the main thread.

Fixes: 363880547
Test: atest CommunalAppWidgetHostTest
Test: verified manually using widgets with lists
Flag: EXEMPT bugfix
Change-Id: I18cf84bd9f36a1faccac74437e0ba2260735a3a8