Register callback outside of constructor

This object is still constructed for CarSystemUI, because it is injected
by dagger at some point in the complicated dependency chain. This
causes some crashes when we try to remove some of the phone specific
layouts from our codebase.

Bug: 154391045
Test: manual (flash + try keyguard in sdk_gphone_x86 and hawk), atest
SystemUITests

Change-Id: Ie272606c54ed632539feab46ddfb85236ac7944a