KeyguardController: reduce executeTransition calls

Executing multiple transitions for the same trigger activity before its
onCreate is done can cause multiple callbacks if the activity is setting
showWhenLocked at runtime.

Test: atest KeyguardTests
Flag: com.android.window.flags.reduce_keyguard_transitions
Bug: 354647472
Change-Id: I6364ccf644836ed6181f1fd565fcf2f6e255f091