Extract four byte sequence modified UTF-8 into separate classes.

The four byte sequence modified UTF-8 is now handled by classes
prefixed with ART, whereas the base classes implements the standard
modified UTF-8 and should be used by new clients in modules.

Bug: 243194720
Test: atest -rerun-until-failure 10 FastDataTest BinaryXmlTest \
      FrameworksCoreTests:XmlTest FastDataPerfTest
Change-Id: I08cea54c2a27249e00b7e155af0cf11949486375