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
Add BackButtonAlphaHandler to set back button alpha, instead of setting
it from multiple places.
Also force back button alpha to be 1 if swipe up is disabled (b/80091187)
Change-Id: I49b63a0e6b033a3a947a847669a398f1b9ff0564
Tony