OpenJDK 11: Merging in java.lang.String (isBlank() method)

This is part of merging upstream changes from OpenJDK 11.28. This CL
adds isBlank() method; and adds unit tests for it.

Bug: 202962059
Test: atest CtsLibcoreOjTestCases:test.java.lang.String.{IsBlank,Strip}
            CtsLibcoreTestCases:libcore.java.lang.StringTest
Change-Id: I387ad19d2ceb8d53fcd2d605627710ab91637bd7
7 files changed