Fix context leak

Using an activity context with AudioManager could cause that context
to be held on to longer than desired, for example if the caller
acquired audio focus but never abandoned it. Fix acquire/abandon in
VideoView, and use the application context in AudioManager to mitigate
the issue for other misbehaving code.

Bug: https://code.google.com/p/android/issues/detail?id=152173
Change-Id: I0fb8390207422c784800dda25b1f2c03d4574bcd