Rename isHeadsUpGoingAway to headsUpAnimatingAway

There is a temporarily state for heads up notifications, when they are
not pinned anymore, but they are still visible on top of the screen for
animation reasons. We refer to this state by two different names from
the code. This CL unifies these namings.

Bug: 325936094
Test: atest SystemUITests
Flag: N/A
Change-Id: I818693757651c28cfb69ced65c07ecbc467ae99a