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