[SPARK-15518] Rename various scheduler backend for consistency

## What changes were proposed in this pull request?
This patch renames various scheduler backends to make them consistent:

- LocalScheduler -> LocalSchedulerBackend
- AppClient -> StandaloneAppClient
- AppClientListener -> StandaloneAppClientListener
- SparkDeploySchedulerBackend -> StandaloneSchedulerBackend
- CoarseMesosSchedulerBackend -> MesosCoarseGrainedSchedulerBackend
- MesosSchedulerBackend -> MesosFineGrainedSchedulerBackend

## How was this patch tested?
Updated test cases to reflect the name change.

Author: Reynold Xin <rxin@databricks.com>

Closes #13288 from rxin/SPARK-15518.