Support auto-complete in PhoneNumberFormattingTextWatcher. Also, simplify logic.

Previously, the assumption was that only a deletion or an insertion can happen
at a time; but with auto-complete, a deletion and insertion happen at the same time.

Needed for bug 5992672 in the Messaging app and is nice to have in the platform.

Change-Id: I2d80cecc486e7a1ceeaeb34866bcd834074f5ead