Prevent keyguard from flashing after dismissing

If the keyguard is dismissed from glanceable hub, it may flash due to
the alpha being temporarily set back to 1f. This change prevents setting
the alpha here since the keyguard should be hidden when on the hub.

Bug: 363093461
Test: atest KeyguardInteractorTest
Flag: EXEMPT bugfix
Change-Id: I6993599d4b620c8fb2565004464c27eb600a260b