Parse escaped sequence strictly in CSV parser

This change alters the CSV parser behavior. The parser only resolves
  * String resource reference. ["@string/res" -> "<content_of_res>"]
  * Other occurrence of escape sequence will be intact. ["\x" -> "\x"]

Before this change, escape sequence in moreKeys string is parsed three
times. At first in parsing string resource, next in CSV parser, and at
last in KeySpecParser. So that representing single escape character
itself is a bit annoying, "\\\\\\\\".

Now we can represent single escape character itself in string resource by "\\\\".

Change-Id: Ib978e17b779cc82585eed8241ac3857508b14bc7