libstagefright: Optimize h263/mpeg4 assambler to reduce frame dropping in the assambler layer.

If current rtp packet lost, when submitting the last access unit to construct a
video frame it will set the last access unit to be damaged and will drop it. It
is the google orignal design. But if the last access unit is a whole frame it will
cause an additional frame dropping. So use "M" marker to detect the frame boundary,
if the last accecc unit "M" is set, we don't drop it, this can improve streaming
video quality when rtp packet dropping too much.

Change-Id: I895ef7fd86fbdc391ba5e84e6dd68ee2fc87e3b8
Author: Feng Xiao<fengx.xiao@intel.com>
Signed-off-by: Feng Xiao<fengx.xiao@intel.com>
Signed-off-by: Yi Wang<yi.a.wang@intel.com>
Singed-off-by: Shuo Gao <shuo.gao@intel.com>
Signed-off-by: Bruce Beare <bruce.j.beare@intel.com>
Signed-off-by: Jack Ren <jack.ren@intel.com>
Author-tracking-BZ: 49120, 23827, 26068