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Creates an observer protocol for AppState and makes MainContorller an
observer. Creates an observer method that is called when the first
scene is activated, to drive one-time UI-related init. Makes MC run
its one-time init from tihs callback.

This prevents a crash where on backgrounding and reopening, the one
time startup is performed again, leading to DCHECKs.

Bug: 1045658
Change-Id: I608975e8690e9c874a1913f7458560a54429d611
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2128509
Commit-Queue: Stepan Khapugin <stkhapugin@chromium.org>
Reviewed-by: Mark Cogan <marq@chromium.org>
Cr-Commit-Position: refs/heads/master@{#754928}