cros: Refactor login_screen_controller a bit.

- Reorder class members to follow styleguide
- Move LockScreenAppsFocusObserver to LoginScreenController::Observer

I am planning on adding more methods to the observer; instead of having a
separate observer for each method it seems cleaner to have one larger observer.

Bug: 784495
Change-Id: I7de65cbc4abae26b23a11344b0a065b73032e09d
Reviewed-on: https://chromium-review.googlesource.com/978833
Commit-Queue: Jacob Dufault <jdufault@chromium.org>
Reviewed-by: James Cook <jamescook@chromium.org>
Cr-Commit-Position: refs/heads/master@{#546235}

• ash/BUILD.gn[diff]
• ash/login/lock_screen_apps_focus_observer.h[Deleted - diff]
• ash/login/login_screen_controller.cc[diff]
• ash/login/login_screen_controller.h[diff]
• ash/login/login_screen_controller_observer.cc[Added - diff]
• ash/login/login_screen_controller_observer.h[Added - diff]
• ash/login/ui/lock_contents_view.cc[diff]
• ash/login/ui/lock_contents_view.h[diff]