Support undo of some programmatic TextView changes

Applications can use TextWatcher.afterTextChanged() to implement
post-edit formatting of text, such as inserting spaces in a credit
card number. From the user's perspective, the insertion of the spaces
is not a separate action, so that change should be merged with the
previous undo operation.

* Force merge undo states for edits that are triggered by callbacks
  after the InputFilter, such as TextWatchers.
* Reset the undo state when the whole field is reset with setText().
* Create separate undo operations for direct programmatic changes to
  the Editable (e.g. directly calling insert).
* Remove part of the non-forced replacement edit merging code. An
  improved version will land in the next CL.

Bug: 19332904
Change-Id: Iba5366a5aadbe3534554b668f8d417250deff505