stagefright: Cancel audioCheck event when setting nativewindow

In html5 playback, setNativeWindow_l() will be called twice
when switching from landscape to portrait mode. If the 2nd
setNativeWindow_l gets the LOCK before proceeding mCheckAudioStatusEvent
posted in the 1st setNativeWindow_l, it will get the infomation
that seeking is completed and won't do seeking in this run.

CRs-fixed: 624469
Change-Id: Ia5c4e373e8a71b907b15b913085600d9481ef982