KeyguardMediaController: Invalidate NSSL only for MediaContainerView

setVisibility method has two major problems:
  1. Both singlePaneContainer and splitShadeContainer triggers visibilityChangedListener. That invalidates NSSL. We should invalidate NSSL only for MediaContainerView.
  2. Null View might trigger visibilityChangedListener. Null View's visibility is going to be null and it is not equal to the new visibility. So, the controller ends up invalidating NSSL.

This CL address these two problems by null check and container type check.

Bug: 298213983
Test: Play song in splitShade and check NSSL invalidation.
Flag: None
Change-Id: I7d75fe57b78efc3903b6d590cc00ee7ccd9e9a90