RFR: JDK-8258381 : [macos] Exception when input emoji using Chinese input method

[Hsiafan]

The convertNSStringToJString function convert NSString to Java String using NewStringUTF，NewStringUTF accept modified-UTF8 encoded str。This fix using UTF-16 encoding for converting.

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Commit messages:
 - fix input UTF-16 surrogate-pairs in macOS

Changes: https://git.openjdk.java.net/jfx/pull/436/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jfx&pr=436&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8258381
  Stats: 2 lines in 1 file changed: 1 ins; 0 del; 1 mod
  Patch: https://git.openjdk.java.net/jfx/pull/436.diff
  Fetch: git fetch https://git.openjdk.java.net/jfx pull/436/head:pull/436

PR: https://git.openjdk.java.net/jfx/pull/436