I need to use additional window containers in a subsequent CL. Rather than
continue to maintain a mapping of kShellWindowId_Foo to ash::mojom::Container,
just use the ash container ids natively.

* Add kInitialContainerId_Property to window_manager.mojom, which is where we
set initial bounds and initial display.
* Remove container.mojom and associated utility functions.
* Replace usage of Container::LOGIN_WINDOW with LockScreenContainer, which is
where the windows end up.
* Ditto for Container::OVERLAY.
* Move ash/common/shell_window_ids.h to ash/public/cpp/

BUG=647412
TEST=compiles, chrome --mash --login-manager shows login dialogs correctly
TBR=reveman@chromium.org for exo

Review-Url: https://chromiumcodereview.appspot.com/2430593002
Cr-Commit-Position: refs/heads/master@{#426161}