AudioFlinger: duplicating output underrun improvements

Several improvements to avoid underruns on OutputTracks:
1) Set the duplicating thread priority to the same as the
spatializer thread.
2) Set the OutputTrack start threshold to the framecount
of its downstream mixer to help synchronizing both outputs.
3) Reduce the wait time on the duplicating thread to one framecount
to give more chances for an underrunning OutputTrack to catchup.

Bug: 412835975
Test: repro steps in bug
Flag: EXEMPT bug fix
(cherry picked from https://googleplex-android-review.googlesource.com/q/commit:08f45936a661f2c0da83adb32dc2c11b0092fb58)
Merged-In: I24becd50c1af483f302710f57a2f04a35d5d64c3
Change-Id: I24becd50c1af483f302710f57a2f04a35d5d64c3