Add support for SystemProperty flags to SysUI.

This will allow the flag flippin' app to change boolean system
properties that launcher, settings, and system server can read.
Some of these flags are already being read between the various systems
and this gives a unified ux for changing the flags.

Bug: 219067621
Test: manual
Change-Id: I7d1a1e758915f43cac114054a2c2e055706ef3f3