Always return a valid index from Rfc822Tokenizer.findTokenEnd()

If an invalid input string ends with a backslash inside a comment
or quoted string, the returned index would be past the end of the
string. In one case this would lead to a runtime exception being
thrown from MultiAutoCompleteTextView.performValidation.

Change-Id: If629372b429716c25cdc25764f088e95d4812d57