stagefright-plugins: Fix eos flush problem.

stagefright-plugins: Fix eos flush problem.

When input stream eos encounter, the output
buffer are not flushed correctly. a null data
must be passed to the ffmpeg decodebr
(avcodec_decode_video2) when the codec is
with flag CODEC_CAP_DELAY.

This fix cts testEOSBehaviorMpeg4 and testCodecEarlyEOSMpeg4
under package android.media.cts.DecoderTest.

This can be simulate on other devices by disable the h/w
mpeg4 omx component.

FEIJ-88

Change-Id: I84ffd7c2a1e4844afe4c891f0440ed4e3c7effe3