Always set AudioTrack callback if it is offload playback.

If the client is not using data callback, aaudio should also set
AudioTrack callback if it is a offload playback. The reason is that the
framework will need to notify the client for stream end when doing
gapless offload.

Bug: 388969394
Test: test_pcm_offload -c2 -po -f1 -r48000 -T40 -E960000 -B
Flag: EXEMPT bugfix
Change-Id: Ia55f162f2888b3a1b7751cc4e4bb3e198979a801