RFR: 8287695: Use String.contains() instead of String.indexOf() in jdk.hotspot.agent

[Andrey Turbanov]

`String.contains` was introduced in Java 5.
Some code in jdk.hotspot.agent 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 jdk.hotspot.agent

Changes: https://git.openjdk.java.net/jdk/pull/8966/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=8966&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8287695
  Stats: 19 lines in 4 files changed: 0 ins; 7 del; 12 mod
  Patch: https://git.openjdk.java.net/jdk/pull/8966.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/8966/head:pull/8966

PR: https://git.openjdk.java.net/jdk/pull/8966