PSS: Make sure shade is dismissed when starting the app chooser

The app chooser is shown below the shade, therefore it is not visible
to the user, when the shade is open.

This issue was happening when starting partial screen sharing from the
Cast QS Tile.

Fixes: 285291564
Test: atest PlatformScenarioTests:MediaProjectionPermissionTest
Change-Id: I6c783ac4e5da62f5071b99d6f8f1d5a92d410fd3