libstagefright: Correct seekFrame computation in AMRExtractor

- In the music app, enable 'repeat current song' option and start
  playback of any AMR-NB audio clip. Now select the progress bar,
  and keep seeking the progress bar and when it reaches the end
  of the file, sometimes it switches to next clip. The expected
  behavior is to repeat the same clip.
- When the seek time is equal to the duration of the clip, the
  seek frame computed is more than the total number of frames
  present in the clip and hence the AMRExtractor returns an
  error which results in skipping to next clip in the playlist.
- Modified the seek frame computation to handle this scenario to
  fix the issue.

Change-Id: Ic00520f5c69da6a90966813b9183275d14d560b0