libstagefright: Fix intermittent pause during tunnel A-V playback

-Media time is used in onVideoEvent for A-V sync. In offload
 playback we only use realtime(time from DSP).
-Media time should be same as real time. If media time is not
 updated with the real time value A-V sync issues are seen.
-Update media time to real time value to avoid intermittent pause
 during playback.

CRs-Fixed: 572060
Change-Id: Ib1123b7554f3508230ca67cd20a68fa32c753057