SDK / exoplayer

With the current ExtTexMgr,

it can happen that

- `x` frames are registered, but haven't arrived yet
- flush
  - need to drop `x` frames when they arrive on SurfaceTexture
  - status is reset to 0 pending, 0 available, drop `x` when frames arrive
- register one frame
  - status is set to 1 pending, 0 available, drop `x` when frames arrive
- flush
  - now the number of frame to drop is reset to `pending - available = 1`
  - but it should be `x+1`

This CL solves the issue by reporting (by running the afterFlushTask) flush completes only after all the pending frames before calling flush are accounted for.

PiperOrigin-RevId: 506310671