Fix some small related to back button alpha

- Skip updating nav button dark intensity if we're setting it manually
  while SUW is running
- There should only be one alpha StatePropertyHolder for the same view
  otherwise when updating the properties it can clobber a previous state

Bug: 204384193
Test: Disable dark mode on SUW and verify nav buttons show
Change-Id: I450c3a5697954d9b464bdd622847beb2d01f3802