Retire PFLAG_ONLY_DRAWS_BACKGROUND.

Per discussion on another CL, this CL retires
PFLAG_ONLY_DRAWS_BACKGROUND because this is over-optimizing and
misleading.  Also setFlags() needs to care both the background
and the foreground.

Bug: 20734520
Change-Id: I57d63e776b5fbad9effd10ad87e683bbb2a6dacd