RFR: 8270380: Change the default value of the java.security.manager system property to disallow

[Weijun Wang]

This change modifies the default value of the `java.security.manager` system property from "allow" to "disallow". This means unless it's explicitly set to "allow", any call to `System.setSecurityManager()` would throw an UOE.

The `AllowSecurityManager.java` and `SecurityManagerWarnings.java` tests are updated to confirm this behavior change. Two other tests are updated because they were added after JDK-8267184 and do not have `-Djava.security.manager=allow` on its `@run` line even it they need to install one at runtime.

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Commit messages:
 - 8270380: Change the default value of the java.security.manager system property to disallow

Changes: https://git.openjdk.java.net/jdk/pull/5204/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=5204&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8270380
  Stats: 24 lines in 6 files changed: 3 ins; 8 del; 13 mod
  Patch: https://git.openjdk.java.net/jdk/pull/5204.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/5204/head:pull/5204

PR: https://git.openjdk.java.net/jdk/pull/5204