audioflinger: Add flag OFFLOAD to track flags

If the playback thread is Offload, add
AUDIO_OUTPUT_FLAG_COMPRESS_OFFLOAD to the track's flags.
This helps the track to be consistent with the others
running in offload thread.

Bug: 341639197
Test:
- run CtsMediaAudioTestCases pass.
- Functional test - use exoplayer application to play multichannel
  AAC content then switches to play Dolby format content, both content
  playback are normal.

Change-Id: I9d9e41492824c9c43065dd723f9cb78d1a476523