Account for various launcher states when setting back button alpha

- Previously we only check for the flag which is always true in the
  normal launcher state, so if the animation starts, we could end up
  writing the back alpha value without accounting for window focus or
  any other states that launcher is in that affects back button
  visibility

Bug: 155129739
Test: Open -1 article and hit back
Change-Id: I42d27a192b23f394f8565a6893ce092603e31654