fix [3223749] media server crashes when switching mode from video capture to still image capture

there was an issue were in some situation SF would call prepare() on hwc
with a NULL handle and never call prepare again.
in this situation, we onw set the SKIP flag to make sure that hwc
won't process this layer and as soon as we receive our first buffer we
trigger a recompute of the visible regions which will end-up calling
prepare() again.

Change-Id: I6b400b2df79712408b9315a9859290c7fcb1609e