mediaplayer: keep more buffers with the BufferQueue

Change OMX buffer allocation policy to allocate
nBufferCountMin + what is required for the BQ.

For the BQ, try to allocate 2 additional buffers than
the minimum undequeued count.

Also account for the fact that BQ may return one less
than the actual minimum undequeued count.

In most cases the resulting number of buffers ends up
being the same as with the previous policy, but we
keep more buffers with the BQ.

Change-Id: I826db8bf7dd333b620299dba60bf1b81b228275d
Bug: 13170236