Fix removing callbacks from KeyguardUpdateMonitor.

Ensure that all access to mCallbacks is done via the handler
thread.

I also ran an auto-formatter on the code on this file, hence a
lot of random whitespace and new line changes.

Bug: 139051615
Test: atest SystemUITests
Change-Id: Ic12281aa6aedca2fe7c0e8399a09cce6ae3126fd