Create widget host views in background thread

Creation of app widget host views requires IPCs to system_server to get
the remote views to show. This change offloads this to a background
thread in order to ensure smooth scrolling of the grid.

Bug: 356393319
Test: flashed device and opened hub
Flag: com.android.systemui.communal_hub
Change-Id: Ia52ba05148505ed95da2de53a2ec289e79e89d4c