Refactored TextView selectable state methods.

There were inconsistencies across methods on how to decide if text is selectable.

Fleshed up textCanBeSelected to support all cases (including non editable textIsSelectable).

onTouchEvent removed from CursorController, test in SelectionModifierCursorController for
editable text is not needed since it is conditionned by an hasSelectionController().

Bug 5016955: protection in offset for text selection

Change-Id: I1e39f7c4a859476e0220e72a4e490b43e60b4a9a