Fix failure at AudioTrack_ListenerTest#testAudioTrackCallback

After cold start, running the test will cause failure. The reason is
that with cold start, the buffer is being processed faster than the
position callback.
Calling the callback earlier at the very start will fix this issue.

Bug: 63056426
Test: adb shell am instrument -e class 'android.media.cts.AudioTrack_ListenerTest#testAudioTrackCallback'
      -w 'android.media.cts/android.support.test.runner.AndroidJUnitRunner'

Change-Id: I7416cecb2837fac4a966e832412948c357e7b8dc