Matcher: Reduce diff vs. upstream OpenJDK 8u121-b13 (refactor part 3).

In this CL:
 - adopt upstream helper methods getTextLength(), getSubSequence()
 - use getSubSequence() to adopt upstream implementation of group(int) and group(String)
 - minor additional tweaks.

This CL does not change behavior.

Bug: 80416774
Test: Treehugger
Change-Id: Id471a4d17d2622591ee299d02b1cfc1324dbd7bf
1 file changed