Add support for using the hub with 3P launchers

We currently read the active launcher from package manager, but a 3P
launcher may not provide the widget picker activity and therefore users
may not be able to edit widgets if they are using a 3P launcher. This
change will always use the system launcher's widget picker, not the
active launcher. This ensures the feature works regardless of the user's
chosen launcher.

Fixes: 361569158
Test: atest CommunalEditModeViewModelTest
Flag: com.android.systemui.communal_hub
Change-Id: Ia97e52b8721110d75ede9f23c7734911c916ceb4