Introduce keyguard transition bug fix feature flag

To fit the plan change, introduce a new bug fix flag for the keyguard
transition change.

Bug: 364930619
Test: KeyguardViewMediatorTest
Flag: com.android.window.flags.ensure_keyguard_does_transition_starting_bug_fix
Change-Id: Ib4199d39adb19cc0fe6061253edadf6d485f1aa0