AXML/ARSC UTF-8 string decoding improvements

I noticed `androguard` doesn't handle all the oddities of AXML/ARSC UTF-8 string decoding (specifically, the presence of surrogate pairs and strings with a length that doesn't fit into two bytes); you might want to take a look at [how my parser handles those](https://github.com/obfusk/reproducible-apk-tools/blob/master/binres.py#L1745).