This commit contains several other changes as part of the upgrade:

- Use -XepAllDisabledChecksAsWarnings to enable all available warnings so
  that we don't need to enable each warning explicitly.

- Remove explicit suppression of warnings in generated code, since
  -XepDisableWarningsInGeneratedCode is now sufficient to suppress all warnings
  in AutoValue and Protocol Buffer generated classes.

- Remove a suppression of "ConstructorLeaksThis", since
  https://github.com/google/error-prone/pull/789 was fixed.

- Fix a few occurrences of "FieldCanBeFinal".

MutableMethodReturnType suggests returning Guava immutable collections from API
methods, but OpenCensus doesn't expose Guava in its API. See
https://github.com/google/error-prone/issues/982.

This commit adds some Nullable annotations that are required by the Checker
Framework, but it doesn't change any other code.  It also suppresses some Error
Prone and FindBugs warnings that conflict with the Checker Framework, since the
three tools use different algorithms.

I used the latest list of bug patterns from http://errorprone.info/bugpatterns.
I had to turn off FieldMissingNullable because of an Error Prone bug and
suppress two instances of other warnings in the code.

I removed NonRuntimeAnnotation and ProtocolBufferOrdinal from the list because
they were promoted to errors. I also enabled two previously disabled warnings
because they seemed to no longer cause any problems in this project.

This commit also fixes two occurrences.