fix empty contact filter
prevent NPE in MMS logic

Fixes #1434
// FREEBIE
Merge pull request #1354 from agrajaghh/fix_empty_contact_filter

Fix empty contact filter not working properly
Merge pull request #1437 from mcginty/mms-npe

prevent NPE in MMS logic
fix receiving utf-8 characters in multimedia push messages

Throw AssertionError instead of logging and trying to recover
Replace XML serializer in plaintext export
Fixes #342
Alternative to #929

- using regex pattern/matcher to escape chars below 0x0020 and above 0xd7ff
- using String.Replace to escape XML entities