SF: add a solid background when hole punching a cached set

When we punch a hole to round the corners on a PIP window we need
to make sure that the cached set is opaque to prevent blending of the
rounded corner edges with random opaque buffers.

Bug: 194307461
Test: PIP window on solid color wallpaper
Change-Id: I5ab80103dc5b43db85c4f12c9efd5ba2a1dfd076