[Partial screen sharing] App selector initial version

Adds an activity that inherits ChooserActivity
to select an app in the partial screen sharing
flow under a feature flag.
It doesn't have a proper visual design, analytics,
enterprise policies handling.

Bug: 218314434
Test: start partial screen sharing
Change-Id: Ia7d9ae07e2ef35d33f85530f26cab4e6cdc1f94a