OpenJDK 11: Merging in java.lang.String (chars() and codePoints() methods)

This is part of merging upstream changes from OpenJDK 11.28. This CL
adds overrides of CharSequenc's chars() and codePoints() methods;
and adds unit tests for it.

Bug: 202962059
Test: atest CtsLibcoreOjTestCases:test.java.lang.String.Chars
Change-Id: I08367092747a93556442683e07c42b503ba488c6
3 files changed