Add Tv Pip rounded corners tray

This CL adds a permanent grey border around the pip surface, which
has rounded corners, and a backing surface, with the same color as the
border. This way the pip surface is sandwiched between the two pip menu
surfaces

Bug: 219883374
Test: m && open app in expanded pip && move pip to opposite edge
    && start dream && wake up dream && toggle expanded pip
Change-Id: Idf22ca22044e7e5e3ca6b39326e3afb76e3ee43c