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This is a follow-up to https://codereview.chromium.org/2519853004/
which made it possible to show the first FRE screen before native
is initialized, to support fetching the variations seed at that
time.

This CL further builds on that to make FirstRunActivity actually
be an AsyncInitializationActivity, so that it never needs to do
the synchronous native init path - instead waiting for async
init to be done before allowing the user to proceed.

When clicking Accept on the first FRE page (ToS and UMA), this
will now show a spinner while waiting for native init and the
variations seed fetch (whose timeout has been lowered to 3s).

BUG=632199

Review-Url: https://codereview.chromium.org/2561433002
Cr-Commit-Position: refs/heads/master@{#442919}