[automerger skipped] audio: return accurate frame count when track is flushed...

[automerger skipped] audio: return accurate frame count when track is flushed am: 8735d0dd am: f0a36003 am: 58721a10 -s ours am: c661faac -s ours

am skip reason: skip tag Change-Id I3b04999a29d85a91b54734514cfa93bc93f55c33 with SHA-1 1ed46a2c is already in history

Original change: https://android-review.googlesource.com/c/platform/frameworks/av/+/1644226

Change-Id: Ib1452ca0f417ae3f9752fcdc4c4b262dd537bef0