[audio][nuplayer] Aggregate buffers to reduce power.

Combine audio data from multiple buffers into one.
This reduces churn between threads and saves battery power.

Only use buffer aggregation when offloading the decoder.
Pend buffer if it won't fit in the bigger buffer.

Bug: 15094301
Change-Id: Iad3dac90b5d6b0fa43432b0e6776652ee2eafaa1
Signed-off-by: Phil Burk <philburk@google.com>