Refactors Sandbox to more easily add new tutorials.

 - Extracts abstract classes for common functionality
 - Renames some layouts, etc. to not be back-specific
 - Consolidates more logic in the controllers rather
   than classes like BackGestureTutorialTypeInfo
 - Removes redundant TutorialStep enum (combining it
   with TutorialType)

Still considering removing additional layers of
abstraction like the Fragment itself (instead
keeping UI within the Activity, which would still
be controlled by a TutorialController).

Test: Built and went through the Back tutorial to
confirm existing funcitonality was not disrupted.
Bug: 148542211

Merged-In: Id893869cb59609141dcdbdca01744d0f5952b546
Change-Id: Id893869cb59609141dcdbdca01744d0f5952b546