Fix bugs don't remove track of DirectOutputThread

Don't remove track of DirectOutputThread by a wrong state control for track stop processor.
Change a state to STOPPING_1 in Track::stop() function, but check STOPPING_2 state in DirectOutput::prepareTracks_l
function.
So the Track is doesn't add tracksToRemove vector.

I thing, change check condition from isStopping_2() to isStopping() function in DirectOutput::prepareTracks_l() function.

Change-Id: I68c4346f02ef1b8635a2180c2a0b3089ce2af955
Signed-off-by: DongKyoon Han <dk.han@lge.com>