[jdk17] RFR: JDK-8270075 SplittableRandom extends AbstractSplittableGenerator

[Jim Laskey]

Random.AbstractSplittableGenerator is an internal class that should not be exposed in a public API.

-------------

Commit messages:
 - Remove public exposure of AbstractSplittableGenerator

Changes: https://git.openjdk.java.net/jdk17/pull/243/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk17&pr=243&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8270075
  Stats: 48 lines in 1 file changed: 29 ins; 1 del; 18 mod
  Patch: https://git.openjdk.java.net/jdk17/pull/243.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk17 pull/243/head:pull/243

PR: https://git.openjdk.java.net/jdk17/pull/243