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This patch adds a LayoutManagerProvider with a static method to access
the activity's LayoutManager. This patch has a fair amount of
indirection to keep certain apis internal and the public api clean.

LayoutManagerAppUtils: A utility class only exposed to app or glue
code to attach the LayoutManager to the window. This exists in its own
build target with ManagedLayoutManager.

ManagedLayoutManager: Another app/glue code api for lifecycle
management. This extends LayoutManager and includes other external
signals that would not otherwise be exposed through the LayoutManager
interface.

LayoutManagerProvider: This is the main access point for clients
wanting an instance of the LayoutManager.

LayoutManagerProvider.Unowned: A package-protected interface for
obscuring attach and detach method implementations in the public api.

Bug: 1161560, 1167255
Change-Id: Ie9561cd6d9dafcca8acac54a85c3c7bc27ca085e
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2634003Reviewed-by: Tommy Nyquist <nyquist@chromium.org>
Reviewed-by: David Trainor <dtrainor@chromium.org>
Reviewed-by: Mei Liang <meiliang@chromium.org>
Commit-Queue: Matthew Jones <mdjones@chromium.org>
Cr-Commit-Position: refs/heads/master@{#846268}