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This patch splits the BottomSheetController into 3 pieces:
  - BottomSheetController: This interface is intended to be the public
    interface for anyone trying to use the bottom sheet and should
    freely be passed around in features.
  - BottomSheetControllerInternal: This interface specifies methods
    that are necessary for "glue" code to effectively manage the
    sheet. When the sheet has its own component, the only code allowed
    to depend on it will be glue.
  - BottomSheetControllerImpl: This is the implementation of the above
    interfaces and will eventually be private (or just internal to the
    bottom sheet widget).

This patch also replaces most of the test-exclusive methods using
various alternatives in tests outside of the bottom sheet's package.

Bug: 1002277
Change-Id: Ic84ae52fab3554ef38d84ee32734340c224d8872
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2225845
Commit-Queue: Matthew Jones <mdjones@chromium.org>
Reviewed-by: Theresa  <twellington@chromium.org>
Cr-Commit-Position: refs/heads/master@{#775278}