Make KeyguardUpdateMonitor#registerCallback in KeyguardStateControllerImpl to...

Make KeyguardUpdateMonitor#registerCallback in KeyguardStateControllerImpl to run on the main thread.

Problem:
The introduction of the per-display component means the KeyguardStateControllerImpl constructor is no longer guaranteed to be called on the main thread, which invalidates a core assumption of the class

Also, gate the eager initialization with the enable display wallpaper presentation flag, which gates the first feature that requires eager initialization.

Test: atest SystemUITests:KeyguardStateControllerTest
Flag: com.android.window.flags.enable_connected_displays_wallpaper_presentations
Fix: 443810777
Change-Id: I0ffb8f7f069302361c3181fa3d6d1f1c581836b5