Merge "Post UserTracker callbacks to background" into main

...temUI/src/com/android/keyguard/KeyguardUpdateMonitor.java

...rc/com/android/systemui/recents/OverviewProxyService.java

...SystemUI/src/com/android/systemui/settings/UserTracker.kt

...emUI/src/com/android/systemui/settings/UserTrackerImpl.kt

...ndroid/systemui/settings/dagger/MultiUserUtilsModule.java

...s/src/com/android/keyguard/KeyguardUpdateMonitorTest.java

...m/android/systemui/settings/UserTrackerImplReceiveTest.kt

.../src/com/android/systemui/settings/UserTrackerImplTest.kt

...tils/src/com/android/systemui/settings/FakeUserTracker.kt