Only buildLayer() on attached views

A recent change to ViewPropertyAnimator.withLayer() builds the layer
immediately after creating it. This works in general, but if the view
is not attached, buildLayer() throws an exception.

The fix is to ensure that the view is attached before calling buildLayer().

Issue #10750925 Dialer crashed and phone dropped while on call

Change-Id: I801c835a0f5cb81e159fe90c157c122cf2d0da01