OpenJDK 11: Merging in java.lang.String (import checkBounds* helpers)

This is part of merging upstream changes from OpenJDK 11.28. This CL
imports checkBoundsBeginEnd() and checkBoundsOffCount() helper methods
and replace repetitive code with these helpers.

Bug: 202962059
Test: treehugger
Change-Id: Ib2b8d0baa898aefeb823a98a1dc4a4e7caacf2d5
2 files changed