Set bg executor to inflate widgets asynchronously

AppWidgetHostView allows the caller to specify a background executor to
inflate RemoteViews on. This ensures expensive takss such as image
loading happen in the background.

Bug: 369210917
Test: verified on glanceable hub with various different widgets
Flag: EXEMPT bugfix
(cherry picked from https://googleplex-android-review.googlesource.com/q/commit:13f60bf537759f959bb944e7644d75813821ce4b)
Merged-In: Ic5225a0b3296548bd918ec70cab54887a64df937
Change-Id: Ic5225a0b3296548bd918ec70cab54887a64df937