Update user icon when dark theme toggled

Previously, after toggling dark theme on/off, the keyguard user icon
would not update until it was clicked.

Also, remove unused private field mDarkAmount

Bug: 184204785
Test: With config_keyguard_user_switch_opens_qs_details=true, toggle
      dark theme on/off and observe keyguard user icon
Change-Id: Ic47ed44c0c970d050178ce2c26aaccb661286ece