Fix several bugs with entering/exiting EditWidgetsActivity.

In particular:

1. Fixes an issue with lockscreen flashing when leaving
   EditWidgetsActivity.

2. Fixes an issue with the notification shade appearing with a
   transparent background when shown over communal hub.

Bug: 325108777, 339421165
Test: atest CommunalRepositoryImplTest
Test: atest CommunalInteractorTest
Test: atest CommunalEditModeViewModelTest
Flag: com.android.systemui.communal_hub

Change-Id: I6d5a6c82ff9ec34907b8ff4a76deb5c2baf857fd