AudioTrack: fix direct tracks not pausing

When a Direct Track is paused and the HAL does not support
pause() and resume() then the HW never gets paused.
The app can just keep writing data, which gets played.

Bug: 18899620
Change-Id: Ice0f360956ff7ca425f6f24a0a2a8640d8b43fa8
Signed-off-by: Phil Burk <philburk@google.com>