Update docs for PhoneNumberFormattingTextWatcher.

1. Deprecate this; it is not maintained and it is a thin wrapper on the
`AsYouTypeFormatter`.
2. Update constructor docs to add WorkerThread annotation since this loads
data from the file system and could cause an ANR.

Flag: Docs only change.
Test: Docs only change.
Fixes: 267915881
Change-Id: I6fa7042010b4d6b2641c06d3d9225fc411a33794