Fixes flickering when transafer splash screen view

...from un-synced transaction.
The transaction for reparenting the child surface within a SurfaceView
can be applied in BlastBufferQueue. This may lead to asynchronicity
with the timing of applying the hide splash screen view leash during
frame commit. Additionally, ensure that the starting window leash is
ready before being sent to the client.

Flag: com.android.window.flags.splash_screen_view_sync_transaction
Bug: 387231234
Bug: 402644135
Test: cold launch Google meet several times and verify no flickering
Change-Id: Iabb63ac37e784f40a788f26f23ebe60dd270eed5