Add support for containing class fields to `RxNullabilityPropagator`

This is to track a known limitation in handling fields from the containing context when dealing with nullability propagation across RxJava filter and map operations.

Here is an illustrative test case that currently produces nullness issues but should be safe:

  private static class FieldFromContext {
      @Nullable public NullableContainer foo;

      private Observable<String> filterThenMapTwoGets(Observable<Void> observable) {
          return observable
                  .filter(unit -> foo != null && foo.get() != null)
                  .map(t -> foo.get().toString());
      }
  }

(add to RxSupportNegativeCases.java)