audioflinger: set flush pending on invalidating offload track

On invalidating an offload track, the IAudioTrack instance is
destroyed and the offload output is released. If it so happens
that APM::getOutputForAttr for the new IAudioTrack is called before
OffloadThread::prepareTracks_l checks and removes an invalid track,
the same output can get reused.

The side effect of this is data present in HAL and below from before the
invalidate will be rendered before data from the new seek position
is rendered. This is unexpected.

To fix this, set hint to issue flush when an offload track is invalidated.

Bug: 28566885
Change-Id: Ib6c38a3abb600598b87591bac90d03b7150d5216