Add basic support for Ctrl-Z to editable TextViews

* Add an UndoManager to the Editor for each editable TextView
* Record operations as being owned by that Editor
* Parcel the undo state
* Wire hardware keyboard shortcuts Ctrl-Z for undo and Ctrl-Shift-Z
  for redo into TextView shortcut handling.
* Expose IDs for "undo" and "redo" for symmetry with cut/copy/paste.

Tests in CTS TextViewTest will land separately in ag/635321

Bug: 19332904
Change-Id: Ibd23843f86b4cd556751faa0b3ad1f35d6d49785