Add explicit method to dismiss Keyguard

The flag is a bit clunky for most cases, and a method is more
clear.

Test: cts/hostsidetests/services/activityandwindowmanager/util/run-test
android.server.cts.KeyguardTests

Test: cts/hostsidetests/services/activityandwindowmanager/util/run-test
android.server.cts.KeyguardLockedTests

Test: runtest systemui -c
com.android.systemui.keyguard.DismissCallbackRegistryTest

Bug: 30961403
Bug: 27422134
Change-Id: I39de90c7cfecd99350a74f72cd76418e337f2b79