Remove mVideoPaused from VideoPreference

- We destoryed the MediaPlayer when VideoPreference is onPause().
When VideoPreference is onResumed, MediaPlayer always start from
pause state, we don't need a data save/restore current video state.

Bug: 143270527
Test: robolectric, manual
Change-Id: I544e933e4237f6d92aeff8a7eb04b52e89d74a4a