RFR: 8287497: Use String.contains() instead of String.indexOf() in java.naming

[Andrey Turbanov]

`String.contains` was introduced in Java 5.
Some code in java.naming 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.

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

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

Changes: https://git.openjdk.java.net/jdk/pull/8938/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=8938&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8287497
  Stats: 13 lines in 4 files changed: 0 ins; 0 del; 13 mod
  Patch: https://git.openjdk.java.net/jdk/pull/8938.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/8938/head:pull/8938

PR: https://git.openjdk.java.net/jdk/pull/8938