audio: Increase kFastTrackMultiplier from 1 to 2

- Fast Track implementation is modified to use single
  buffer in kitkat. This is causing audio breaks when
  long DTMF tones are played
- Fix is to use double buffering to avoid audio breaks

Change-Id: Idc811ee7fe0d7f49bfa95ad34bc6e88bd04b734e
CRs-Fixed: 561567
audio: Use composition instead of multiple inheritance

- AudioTrack is derived from BnDirectTrackClient along
  with RefBase to receive the notifications from
  AudioFlinger in tunnel/LPA mode. This is breaking
  binary compatibility for the clients which use native
  AudioTrack directly instead of Java based AudioTrack
  API
- Fix is to use composition here instead of inheritance

Change-Id: I9fd2c61cff5629924e3b08f63559679e0263bcfd
CRs-fixed: 607574
Merge remote-tracking branch 'omni/android-4.4' into android-4.4