[SPARK-9261] [STREAMING] Avoid calling APIs that expose shaded classes.

Doing this may cause weird errors when tests are run on maven, depending
on the flags used. Instead, expose the needed functionality through methods
that do not expose shaded classes.

Author: Marcelo Vanzin <vanzin@cloudera.com>

Closes #7601 from vanzin/SPARK-9261 and squashes the following commits:

4f64a16 [Marcelo Vanzin] [SPARK-9261] [streaming] Avoid calling APIs that expose shaded classes.