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This CL adds an animation that keeps the resuming Activity hidden for 500ms while 
starting up to allow time for us to draw the black overlay, also added in this CL,
and prevent 2D UI from being shown while entering VR.

This also performs some cleanup around VR entry (now that it's even more async than
before) to ensure that we don't continue trying to enter VR after we try to exit
VR (like when the activity is paused, etc.).

Bug: 728148
Change-Id: I344ad661b632bde74764b134e9d9f70f6258d68d
Reviewed-on: https://chromium-review.googlesource.com/519483Reviewed-by: David Trainor <dtrainor@chromium.org>
Reviewed-by: Yash Malik <ymalik@chromium.org>
Commit-Queue: Michael Thiessen <mthiesse@chromium.org>
Cr-Commit-Position: refs/heads/master@{#476301}