Set alpha to 0 on attach to avoid flicker at transition start.

Otherwise there is a race condition risk of rendering it for one frame
before it actually gets the first call to onAnimationUpdate.

Change-Id: Ifcf7103a78631c6c1408735acf3eb13888ae8dea
Test: manual, 20x enter Maps PiP without the repro of overlay flicker
Bug: 281711939