Pretend to be having the original background

...as otherwise setWindowBackground(bg) => bg == dv.getBackground
property is violated.

Also ensure the setBackground updates mOriginalBackground as well,
such that getBackground doesn't return a stale value.

Test: WindowTest
Test: Open maps
Bug: 130586696
Fixes: 130170137
Change-Id: I22b74ca6d4fd18600645b5d24814a90b8528745c