Make RuleContext.getConfiguration() private.

This further discourages the paradigm of rule implementations consuming
entire configs vs. declared fragments.

It also makes RuleContext a more reliable tracker of which fragments
rules use.

PiperOrigin-RevId: 184355968

src/main/java/com/google/devtools/build/lib/analysis/RuleContext.java

@@ -453,7 +453,7 @@ public <T extends Fragment> boolean isLegalFragment(Class<T> fragment) {
     return isLegalFragment(fragment, NoTransition.INSTANCE);
   }
 
-  protected BuildConfiguration getConfiguration(ConfigurationTransition transition) {
+  private BuildConfiguration getConfiguration(ConfigurationTransition transition) {
     return transition.isHostTransition() ? hostConfiguration : getConfiguration();
   }