Switch to using a ThreadPoolExecutor for widget inflation on the hub

This matches the behavior of launcher and speeds up initial widget load
times a bit.

Bug: 369412569
Test: verified on device with flag on and off
Flag: com.android.systemui.communal_hub_use_thread_pool_for_widgets
Change-Id: Idab2ebdfb40e08b7a82989f22b85b47a86eb4379