Allow activities on secondary displays to dismiss keyguard

Currently, KeyguardController only looks for activities on the default
display to determine if it should dismiss the keyguard due to the
presence of the FLAG_DISMISS_KEYGUARD window flag. This allows
activities on secondary displays to dismiss the keyguard.

Bug: 64527423
Test: android.server.am.KeyguardLockedTests passes
Change-Id: Icd50e8cdccf63dae6bae1120ca790cf5d4f40674