See  issue 728148  for context.

Brief summary:
Android really likes to show stale screenshots of your app when it's resuming. We really don't want to see these in VR.
So, we add an animation to hide the activity as it's loading, and add a temporary black overlay as the VR UI loads.
However, the animation to hide the activity cannot be cancelled as far as we know, so we have to guess in advance how long it's going to take the app to start up. If we guess a value too low, then we see the 2D UI, and if we guess a value too high then we're adding unnecessary latency.

Ideally we would have a way to tell Android 'don't draw anything until the app does its first post-start paint'.