ADT XML String Refactoring: fix refusing to edit @+id/blah.

It correctly only refuses to edit @string/blah now.

This CL also does a bit of refactoring; I extracted some methods
and a class to make it a bit easier to read.

BUG 2066460

Change-Id: I95a34d28d6390ab0cc075f05ea83ceec04993ae9
2 files changed