Fix undo of text pasted into TextView with TextWatcher

Pasted text is directly inserted into the TextView buffer, so it looks
like a programmatic insert to the undo system. This is fine, but we
need to be sure that subsequent modification by a TextWatcher (for
example, to add spaces to a credit card number) is merged into the same
undo operation -- from the user's perspective, pasting in the text and
having spaces inserted are a single operation.

Also clean up mForceMerge flag in EditOperation -- we don't need to
store it across edits, just use it when a new edit comes in.

CTS test to follow.

Bug: 19332904
Change-Id: I7b3c97c08b83faebeab9f1708b0d6eac6d151d50