Fix SoundPool and MediaPlayerService buffer overflow

Overflow occurs when SoundPool sample tracks cannot
fit in the MediaPlayerService AudioCache buffer.

Unnecessary decoding occurred with AwesomePlayer and
an assert failure occurred with NuPlayer.  NuPlayerRenderer
is also tweaked to handle the latter case.

Bug: 17122639
Change-Id: I4d25d3e2c0c62e36a91da6bf969edabddc2ebbb0