Don't allow AudioTrack frameCount to decrease

This is a workaround for bug that client can cache return value of
frameCount(), and is not notified when this value changes due to automatic
re-recreation of the underlying IAudioTrack.

A better long-term fix would be to notify clients when these kinds of
parameters change, and to fix assumptions in client code that they are
constant (e.g. in SoundPool and maybe obtainBuffer).

Also, once a fast track request is denied, don't request it again.

Bug: 6431187
Change-Id: I55b4ff30bbd9ed3a402e39452a38de52cdea53a9