Enable OvershadowingSubclassFields checker

- Overshadowing fields of superclass can causes confusion and errors
and is probably a good thing to avoid it.

Change-Id: If73e4694ba57bf048495650b047f1943c9a5e3b1
diff --git a/build.gradle b/build.gradle
index e50073d..6dadb5c 100644
--- a/build.gradle
+++ b/build.gradle
@@ -18,7 +18,8 @@
     '-Xep:ClassCanBeStatic:WARN',
     '-Xep:OperatorPrecedence:WARN',
     '-Xep:RemoveUnusedImports:WARN',
-    '-Xep:MissingOverride:WARN']
+    '-Xep:MissingOverride:WARN',
+    '-Xep:OvershadowingSubclassFields:WARN']
 
 apply from: 'copyAdditionalJctfCommonFiles.gradle'