[SPARK-23795][LAUNCHER] Make AbstractLauncher#self() protected (apache#341)

Author: dansanduleac

launcher/src/main/java/org/apache/spark/launcher/AbstractLauncher.java

@@ -274,7 +274,7 @@ public T setVerbose(boolean verbose) {
   public abstract SparkAppHandle startApplication(SparkAppHandle.Listener... listeners)
     throws IOException;
 
-  abstract T self();
+  protected abstract T self();
 
   private static class ArgumentValidator extends SparkSubmitOptionParser {
 

launcher/src/main/java/org/apache/spark/launcher/InProcessLauncher.java

@@ -77,7 +77,7 @@ public SparkAppHandle startApplication(SparkAppHandle.Listener... listeners) thr
   }
 
   @Override
-  InProcessLauncher self() {
+  protected InProcessLauncher self() {
     return this;
   }
 

launcher/src/main/java/org/apache/spark/launcher/SparkLauncher.java

@@ -453,7 +453,7 @@ private ProcessBuilder createBuilder() throws IOException {
   }
 
   @Override
-  SparkLauncher self() {
+  protected SparkLauncher self() {
     return this;
   }