audioflinger: fix offload track transition

Make sure that when transitioning from one active
offloaded track to the next we flush DSP content
if both tracks are not on the same audio session.
This happens when switching between two applications
playing the same type of content (e.g MP3, stereo, 44.1).
In this case, we reuse the same output thread because the track
formats are compatible.

Bug: 11247103.
Change-Id: I2b9031591149adeb70766da5e0d21ff2933a37e8