NuPlayer: Call video flush on audio teardown

when audio tear down happens, audio gets flushed and seek is issued.
If video is not flushed before seek, previous video buffers will be
rendered and leads to A/V sync issue. Hence call video flush.

Author: Cong Jiajia <jcong@codeaurora.org>

Bug: 70919418
Test: MediaPlayerTest
Change-Id: I01f744c1eae99a1d0f81ca84f536d493a2a54b01