Respects explicit formatted="false" tag in strings

[bkeller-vuzix]

It is valid for Android XML files to have strings that disregard the formatting characters (like percent) by setting the tag formatted="false", such as:

<string name="discount" formatted="false">50% off</string>

This tag was previously not being carried through an export/import cycle. Now it is tracked properly. In the po file we will store the flag "no-c-format" to mean formatted="false" in the XML.

Note: this is only implemented for "string" at this time, not "plurals" and "string-array".

Closes #24 #26