Make single bounding rect when none set relative to window origin

When a source doesn't have any bounding rects set it means that the
entire source frame should be treated as one single rect. This change
makes that rect relative to the |relativeFrame|, which is what the API
is promising.

Bug: 328667331
Test: atest FrameworksCoreTests:InsetsSourceTest
Change-Id: I5669895f15bfc6e1e8dad8aa32191e48b06bfbbb