aaudio: offset MMAP timestamps

This can be used to debug and analyze problems
with the AAudio MMAP IsochronousClockModel.
That is a model of the DSP that is used by AAudio
to predict the read/write timing of the DSP.
If there is an error in the model or in the timestamps
then the DSP and CPU pointers can cross, which causes
an audio glitch.
By changing the offset we can force glitches and indirectly
measure the time distribution of the DSP transfers.

Test: adb shell setprop aaudio.out_mmap_offset_usec
Bug: 123096058
Change-Id: I9a3df345a6820baf20a24d261642b3c8c0c2a27e