In BufferedReader.readLine and other similar scenarios, we need to use StringBuilder.append(char[]) to build the string.

~RogerRiggs completed the optimization when the encoder is LATIN1 in PR #24967. This PR continues to complete the optimization when the encoder is UTF16.

For these scenarios, we can use Unsafe.copyMemory instead of the character copy of the char-by-char loop to improve the speed.