Suspend when draining MMAP offload stream.

For MMAP offload playback, when the clients have written big enough
data, it makes sense to suspend the callback thread and the command
thread at the service side to drain as much data as possible. This can
help save battery.

Bug: 426614937
Test: atest AAudioTests
Test: play offload streams
Test: test_pcm_offload -f2 -c2 -r48000 -T20 -E48000
Test: test_flush_from_frame -f2 -c2 -r48000
Flag: EXEMPT bugfix
Change-Id: I28e7b57892ecd183362cf2f3b715e0c03055760b