Fix density change bugs in SystemUI

- Reinflate KeyguardUserSwitcher on density changes.
- Update size of UserAvatar on Keyguard.
- Fix brightness slider width in quick settings.
- Refetch and rescale user icons on density changes.

Change-Id: I961c59b8b224d6ac22cbbb8e115b8a12547a04b9
Fixes: 28535987
Fixes: 28454340