MPEG4Writer:changed logic when setting last frame duration explicitly

Apps are using zero length frames with EOS flag at the end of a track.
However, they are either ignoring timestamp or are setting to zero.
This breaks the CHECK for last frame duration to be positive and media
file is not created at all. Inorder to let apps to compose movie file in that
case, changed the logic of that code to care only when timestamp of EOS
flagged empty buffer is positive. Also, throw only a warning message when
last frame duration becomes negative.
In other cases, old behaviour of copying the previous frame's duration
for the last frame would stay the same.

Bug: 151056554

Test: 1) The issue mentioned in the bug is not happening with this fix.
      2) atest android.media.cts.MediaMuxerTest
      3) atest android.media.cts.MediaRecorderTest
Change-Id: I1754bb524049a00def232ae82d08fe94fa13bd0f