libstagefright: Handle offload fallback for WAV content

- For WAV content there is no OMX Source, use the AudioTrack
  as OMX Source for fallback.

Change-Id: Ifeaeb2cf8dd777d167b74eebd11803b7f130e857
audioflinger: remove unecessary call to HAL

On a flush, OffloadThread resets its state about HAL being paused.
For the sake of consistency, it is now expected that after a flush
as HW is no longer paused, the resume call is unnecessary.

Change-Id: I47d8b685dd23427bb44e24593124b4e42e4a315e