Reapply back button alpha at end of state animation

We determine the toAlpha at the start of the state change, but something
may happen (e.g. we lose window focus) during the transition. So to make
sure we don't end in a state with the wrong alpha, reapply at the end.

Bug: 109835691
Change-Id: I271a1d269f861f698efbb8e66148a924c8620e71