Use UnlockMethodCache#canSkipBouncer in user switcher

KeyguardMonitor#canSkipBouncer was not updated properly when the phone
was unlocked using fingerprint.

This CL removes that method and changes UserSwitcherController to query
UnlockMethodCache directly, as it was KeyguardMonitor's only client for
that method.

Test: manual unlocking with FP and with pattern
Test: no automated test yet
Bug: 140486529
Change-Id: Idbff4fbabca962c632ff5d78b25418c0502db9a7