Improvements to TextView Ctrl-Z undo support

* Undo/redo restores cursor position
* Multiple deletes are consolidated into a single operation
* Don't create undo history for invalid or no-op operations
* Move logic for merging operations into Editor.EditOperation

CTS tests in android.widget.cts.TextViewTest will land separately.

Bug: 19332904
Bug: 19450037
Change-Id: I9c7887aecdd0209e783fb7cda4cb7081a97a98cd