OpenJDK 11: Merging in java.util.StringJoiner

This is part of merging upstream changes from OpenJDK 11.28. This CL
merges java.util.StringJoiner

Test: m droid
Test: atest StringJoinerTest
Change-Id: I0b075a6c824dceb29af43b4d5b3e92cae53309d7
1 file changed