Defer remove splash screen while device is locked

...and activity does not request showWhenLocked.
The splash screen won't contains secure information, so it's safe to
declared as showWhenLocked. But before remove starting window, if the
activity does not request showWhenLocked and device is locked, try to
trigger unoccluding animation, and keep app window hide until transition
animation finish.

Bug: 378088391
Flag: com.android.window.flags.keep_app_window_hide_while_locked
Test: run simulate app repeatly, verify the app content won't be visible
during transition animation.

Change-Id: Ia2ddece125521eefb15d67e22ea863dfae6af112