Fix doze bug in keyguard user switcher

Previously, the view would be set to GONE at the incorrect time.

KeyguardQsUserSwitchController has no need for knowing the doze amount
at all since it does not need to update text color, so remove the
listener entirely.

Also, add javadoc and update variable names in KeyguardSliceView's
setDarkAmount to better reflect their meaning.

Test: Build and flash blueline-userdebug with QS user switcher enabled
      (config_keyguard_user_switch_opens_qs_details), switch between
      device owner and guest repeatedly. Turn screen off and on.
Bug: 181776379
Change-Id: I772612d49f2e3bcbd6aba334afee17fcb1c84c57