StaggeredGridLayoutManager gap handling bug fixes

SGLM may fail to detect a gap if it is created for not having
enough items while filling towards HEAD. Although it could be
fixed by detecting the case and setting a flag, running a
full gap check at the end of a post layout seems more stable.

Bug: 18970779
Change-Id: Ie267092335ce74d315b4677af78c3adf3a98af61
4 files changed