Add centralized DisplayTracker for SystemUI

In multi-user multi-display systems where there could be multiple
SystemUI instances running, the concept of what displays are relevant
for a particular SystemUI may change. By adding a DisplayTracker class
to abstract away display retrieval and callbacks, these instances can
return the information that is most accurate for them.

Bug: 256236871
Test: atest SystemUITests
Change-Id: I51c1a380a344fb45d5e0de2c5c6ba5610733a3a2