* Implement CurrentStatsState using atomic variables.

* Ignore findbugs warning and add changelog comments.

This commit replaces the most commonly used precondtion checks with methods in
a new utility class, io.opencensus.internal.Utils.

This commit adds an "s" to TimeUtil, StringUtil, and BaseMessageEventUtil, for
consistency with utility classes that have been made public, such as
ContextUtils.

This would have caught the unused variable that was fixed by #1082.

Changes in this commit:

  - Enable FindBugs in tests in build.gradle.

  - Fix some existing minor FindBugs issues in tests.

  - Suppress some warnings in all test classes that are less important in test
    code, such as requiring nested classes to be static whenever possible.

  - Enable nullness-related FindBugs warnings in tests, since we don't run the
    Checker Framework on test code.

* Move BaseMessageEventUtil to trace package.

* Move BaseMessageEventUtil to trace.internal. Work around findbugs.

* Update findbugs-exclude and remove since tag.

The warnings are redundant with the Checker Framework's warnings, and they
sometimes conflict.