RFR: 8274949: Use String.contains() instead of String.indexOf() in java.base

[Andrey Turbanov]

String.contains was introduced in Java 5.
Some code in java.base still uses old approach with `String.indexOf` to check if String contains specified substring.
I propose to migrate such usages. Makes code shorter and easier to read.

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Commit messages:
 - [PATCH] Use String.contains() instead of String.indexOf() in java.base
 - [PATCH] Use String.contains() instead of String.indexOf() in java.base

Changes: https://git.openjdk.java.net/jdk/pull/5559/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=5559&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8274949
  Stats: 40 lines in 17 files changed: 0 ins; 3 del; 37 mod
  Patch: https://git.openjdk.java.net/jdk/pull/5559.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/5559/head:pull/5559

PR: https://git.openjdk.java.net/jdk/pull/5559