Remove patch from AbstractStringBuilder

Remove the last local patch from AbstractStringBuilder by
restoring original method signatures to String and adding
two new private static methods. String already has many
differences from upstream code so it makes sense to keep
the differences there.

After this change AbstractStringBuilder is identical to
OpenJDK8u121-b13.

Bug: 74379469
Test: CTS: run cts -m CtsLibcoreTestCases
Change-Id: I740086e9d9f26474b51e330856256e2f16892f0e
2 files changed