Move keyguard shell transition init to onCreate

Some state isn't available in the Constructor and generally we're
not supposed to be doing much work in constructor anyways vs
onCreate.

The problem here is the base context isn't initialized at
KeyguardService construction time, so we can't get sysui's
application thread.

Bug: 183993977
Test: device doesn't crash on boot with shell-transit enabled.
Change-Id: I4c0d99a913080c767c913168f226f4c688ef8250