Release the media player when the video preference is not visible.

Mediaplayer is not released when the device is paused, and hence it
keep consuming battery.

Fixes: 120945749
Test: adb root && watch -n 0.5 -d "adb shell cat /sys/kernel/debug/msm_vidc/core0/inst*/info\|egrep -i \"height\|width\|instance\|fps\|name\""
      Use above command to check if any decoder instance exists.

Change-Id: Ia0edbba0c2fd3c70753bc36d23d82c0770f95672