Initialize UserTracker with boot user

UserManager provides getBootUser(), which can be used to set
the initial value of the UserTracker, rather than always
default to user 0. This can help keyguard (and others) start
in the right state and reduce jank on boot with HSUM.

Bug: 268416867
Bug: 361730315
Test: atest UserTrackerImplTest
Flag: EXEMPT bugfix
Change-Id: Ie5cae1a5114647b5071cb9587faa4455ed4fc377