Add ShadeController to KeyguardViewMediator

ShadeController is not used yet, but adding the member var as a
standalone CL will allow ag/20355131 to automerge without conflicts
and save us some code review effort.

Bug: 249277686
Test: presubmits
Change-Id: I9cb717410e32c1f2da2711c91f532ab05c0b410b
Merged-In: I9cb717410e32c1f2da2711c91f532ab05c0b410b