MediaMuxer:allow negative presentation timestamps

Observed a few audio files starting with negative timestamps, inorder
for those files to be muxed as is removing the check that was erroring when PTS<0.

Bug:142580952

Test: 1) atest android.media.cts.MediaMuxerTest
      2) atest CtsMediaTestCases -- \
         --module-arg CtsMediaTestCases:size:small

Change-Id: I693f1c0046574c14bf6b5e656c40ed6b73bd0f0e