Migrate UnicodeString usage to equivalent C API

Migration notes:
- 0xfffd is chosen as the substitution character because
  it's current default substition in UnicodeString

Bug: 138994281
Test: atest --test-mapping libcore/luni/src/main/java/org/apache/harmony/xml:presubmit
Change-Id: I37139236350aea69074b7022389f83ba1ed528ed
1 file changed