Fix divider bar floating on keyguard panel

After migration, we use KeyguardUpdateMonitor.registerCallback() instead
of using KeyguardStateController.addCallback() to monitor keyguard's
visibility changes. Since KeyguardUpdateMonitor recrods registered
callbacks with weak reference, the callbacks might be GCed if there's no
other reference.

Fix: 168271350
Test: atest WMShellTest
Test: check divider bar disappears everytime when keyguard showing
Test: check it'll leave one handed mode everytime when keyguard showing
Change-Id: I23ca7e40c59211cbeafb09105f95ae2d1a919031