Hide zoom buttons when a view gets hidden, i.e. by a TabHost

When a View uses a ZoomButtonsController, it needs to explicitly hide
the zoom controls when it gets hidden itself, such as when it's within
a TabHost.  Add a note to the javadoc to do this via
onVisibilityChanged(), and update WebView to follow the instructions.

Thanks to Ross Levinsky for discovering and tracking this one down.

Bug: 2582718
Change-Id: Ia0c9ea4efcb29eb1f321be54f3d8ae1e056df6b7
CC: rosslevinsky@google.com