Re-enable the undo demo activity

The UI now shows these input fields:
* A normal field with buttons for programmatic SetText, append and insert.
* A field with undo disabled.
* A field with a length limit (via InputFilter).
* A field with a credit card input formatter (via TextWatcher)

Bug: 19332904
Change-Id: I41df56c84dd5232918e7ee5e6ed5874b2023cc78
4 files changed