AudioFlinger: Make Direct AudioTrack behavior more flexible for underrun

Allow more time for data to arrive ~200ms.
Push remaining track data in underrun.
Document direct track underrun handling.

Test: directed test (see b/186596553)
Bug: 34711969
Bug: 157682459
Bug: 161089206
Bug: 186596553
Merged-In: I9cd7f1f89287c180c30bba35e73d8d3fd6296eeb
Change-Id: I9cd7f1f89287c180c30bba35e73d8d3fd6296eeb