mediaplayer: optimize buffer queue management

Various changes for power consumption including:
    Restrict the number of messages in flight.
    Buffer more frames in the GenericSource so reads occur in a burst.

Bug: 15094301
Change-Id: I783481fd91f3fdd445b95e88ab82178f649f1a38
Signed-off-by: Phil Burk <philburk@google.com>