API cleanup and purdy-fication.

- Move gesture initiation into TouchInputHandler (out of the Callback,
    it shouldn'ta been in the callback in the first place.)
- Make {Band,Gesture}SelectionHelper implement OnItemTouchListener directly.
- Minor cleanups in GestureSelectionHelper logic.
- Allow SelectionPredicate to answer the "can select multiple" question.

Bug: 64847011
Test: Added gesture initiation coverage.
Change-Id: Ic3b3e8613e6408ad0c611d3224b87ee9330183b7