Fix warnings.

Author: jsuereth

api/src/main/java/io/opencensus/trace/unsafe/ContextHandleUtils.java

@@ -83,6 +83,7 @@ public static Span getValue(ContextHandle context) {
    *
    * @return The context, or null if not a GRPC backed context handle.
    */
+  @Nullable
   public static Context tryExtractGrpcContext(ContextHandle handle) {
     if (handle instanceof ContextHandleImpl) {
       return ((ContextHandleImpl) handle).getContext();

api/src/main/java/io/opencensus/trace/unsafe/ContextManagerImpl.java

@@ -31,11 +31,13 @@ public ContextHandle currentContext() {
   }
 
   @Override
+  @SuppressWarnings({"deprecation"})
   public ContextHandle withValue(ContextHandle contextHandle, @Nullable Span span) {
     return wrapContext(ContextUtils.withValue(unwrapContext(contextHandle), span));
   }
 
   @Override
+  @SuppressWarnings({"deprecation"})
   public Span getValue(ContextHandle contextHandle) {
     return ContextUtils.getValue(unwrapContext(contextHandle));
   }