NuPlayerRender: fix a/v sync issue when playback only video track clip

If only video track exists, mAnchorTimeMediaUs retrieve system time asa/v sync time base.
If playing is paused, we should clear mAnchorTimeMediaUsand reset it when resume playing

Change-Id: Ia517079e0ed1b196c78e08eb2d8c132561beec86
Author: Yi Wang <yi.a.wang@intel.com>
Signed-off-by: Yi Wang <yi.a.wang@intel.com>
Signed-off-by: Feng Xiao <fengx.xiao@intel.com>
Singed-off-by: Shuo Gao <shuo.gao@intel.com>
Signed-off-by: Bruce Beare <bruce.j.beare@intel.com>
Signed-off-by: Jack Ren <jack.ren@intel.com>
Author-tracking-BZ: 48710, 30011