Request a traversal when a saved surface gets redrawn

When a saved surface gets redrawn, we need to return true in
finishDrawingLocked, so that a traversal is requested. This is
needed to update allDrawn/allDrawnExcludingSaved. Some delayed
window removal may be waiting for these flags.

bug: 28797800
Change-Id: I0b58b356e9c580422eb3ff81e8afb2a164cf6e43