This commit improve MediaCodec performances and remove polling every 10 msecs
when DecodeCommon is waiting for an input or an output buffer.

One new thread is used for output: once an input buffer is queued, this thread
will wait indefinitely for a MediaCodec output buffer (this can be cancelled by
a flush).  This buffer will be queued to the video or audio output via
decoder_QueueVideo or decoder_QueueAudio.

This thread, and pf_decode call are locked by the same mutex. Only
mc->dequeue_in/mc->dequeue_out are not locked since these functions can block.

Fixes #15079