Make FastXmlSerializer more suitable to persist arbitrary strings

- Encode '\u000' - '\u001F', so KXmlParser can read them properly.
Otherwise KXmlParser will ignore CRs/LFs in attributes, and CRs
in text.

- Originally FastXmlSerializer would throw if a string contains
dangling surrogate pairs.  Now we REPLACE them with.

Bug 27792649

Change-Id: I10c547dad2475b68f60e9e8208d9a3eae8e20063