Limit black background to only base and starting windows

Show black background behind nav bar only behind windows
with type TYPE_APPLICATION_STARTING or TYPE_BASE_APPLICATION,
which corresponds to main window of an activity.

Bug: 64711770
Test: Go to Wallpapers app, select a static one, click "set" and check the background of the dialog.
Test: Open letterboxed app, go to/from Recents.
Change-Id: I42357a5df70b5dd58b422f31704c71d36074a50c