wm: expose isKeyguardShowing() in WindowManagerInternal

- Right now, the KeyguardLockedStateListener callback argument has a
  different source (mPolicy.keyguardOn) from KeyguardManager.isKeyguardLocked().
- This change allows client to read the correct initial state through
  WindowManagerInternal API

Test: manual
Flag: EXEMPT, refactor (pure structural change)
Bug: 414604681
Change-Id: Icc9c1aed479bcbe62d83e67b0c6d0597ddf883e4