Bug 4231349 fix error in javadoc for android.media.MediaPlayer

In the javadoc for MediaPlayer.OnBufferingUpdateListener,
the percent parameter for onBufferingUpdate() is described as:
  the percentage (0-100) of the buffer that has been filled thus far

This is incorrect. This percentage is the ratio of (current position
+ content buffered) relative to the total duration.
Also buffering updates are only sent for progressive HTTP download.

Change-Id: I9f346a150b39fa1f7b17126119ecc0d7d27fbfbe