[Partial Screensharing] Add app selector activity screenshot test

Adds a screenshot test for the app selector activity.
It also contains changes to utility rules and classes
to allow capturing the whole activity instead of a single
view.

Bug: 235465652
Test: atest com.android.systemui.mediaprojection.MediaProjectionAppSelectorScreenshotTest
Change-Id: I558c3d0e8893091d42f90cb0358c9fd030cbc0dc