Segregate Mouse and Touch input handling.

- The change also isolates these respective classes from DocumentsUI
    by the introduction of ItemDetailsLookup && ItemDetails.
- Replace DocumentDetails w/ ItemDetails.
- Introduce a pattern in the code of using input-type specific callbacks
    (one each for mouse, touch, and keyboard)
    in order to allow "openDocument" behavior to be specialized.
    DocumentsUI has some nuanced behaviors around opening that weren't
    appropriate for inclusion in support lib.
- Move focus related access to callbacks...focus is outside of the scope
    of this change (and likely this entire effort).
- Cleanly separate gesture event delegation.
- Address outstanding commments from: ag/2924928

Change-Id: Ibc26bb66ac805cd1f2345e7f01cedcf865c8467d
Bug: 64847011
Test: Passing (and substantially massaged to support the refactoring).