Fix flickering when starting and ending apps.

Surfaces were displaying animations in their entirety for a single
frame before the animation kicked in. This caused a flash on the
screen. By setting the animation to invisible (alpha=0) at their
start it makes no difference if they are displayed.

Fixed bug 6176540.

Removed redundant mDimAnimator.show call.

Change-Id: I47c1b0d38273b011d9115822a8476671d6a050fc