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: I79fe3764edce5a0acb8fae146905d20e77946823
Author: Feng Xiao <fengx.xiao@intel.com>
Signed-off-by: Feng Xiao <fengx.xiao@intel.com>
Signed-off-by: Dan Liang <dan.liang@intel.com>
Signed-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