Add tests for custom actions to TvPipMenuTests

Test that correct UI elements are displayed for the custom actions; that
the set of custom actions can be updates (while PiP menu is shown); that
custom actions collaborate correctly with "regular" actions (Full screen
and Close), as well as with the media contol actions.

Bug: 171520419
Test: atest WMShellFlickerTests:TvPipMenuTests
Change-Id: I1f5b1e4896a60b8d2bfb118e548466e7f4036e61