Isolate Keyguard from main thread badness

Adds a bindService variant to run the ServiceConnection callbacks
on a dedicated Handler.

Changes KeyguardService binding to use the foreground thread
instead of the main thread and changes state to the
KeyguardScrim on the UI thread.

Bug: 26954967
Change-Id: I9d7bd85382816cd0e23772b14ff6518266a9d232