Fix pretty JSON when strings end with backslashes

If a string encoded as part of the output ends in a backslash
(e.g. an edit token), FormatJson::prettyPrint() may incorrectly
treat the unescaped double quote marking the end of the string as
a character that is part of the string.

This is a serious problem in that the "pretty" output may not
necessarily be valid JSON; a later string literal might contain
one or more of these tokens: :[{,]}

To fix the bug, I exploit strtr's behavior when it is given an
associative array having keys of the same length to skip over
escaped backslashes while replacing escaped double quotes with "\x01".

I also updated the corresponding unit test.

Change-Id: I159105b6493c14b82cd0a41a95e04bfed744931e
(cherry picked from commit 217cb2e3a6ea805eee4cc81b1cb43562620f88b3)