audioflinger: fix mmap interface callback lock

Do not hold the MmapThread mutex when calling methods on
MmapStreamCallback as AAudioService will synchronously call
methods on MmapStreamInterface that will in turn acquire the mutex.

Bug: 78118584
Test: 'adb shell write_sine_callback -m3 -pl -s100 -x' and Plug headphones in and out.
Test: AAudio CTS tests

Change-Id: I5953bb449e67735271203a2e1dbef0a2b451f5d3