Enable tradefed error_prone rules on contrib too

Ensure we maintain the basic same standard in TF.

Test: build with error prone
Change-Id: Ib745cde06ba6bc7275ba0ce5af763a057e968d4b
diff --git a/Android.mk b/Android.mk
index 744e0a0..afc2ddf 100644
--- a/Android.mk
+++ b/Android.mk
@@ -22,6 +22,7 @@
 LOCAL_JAVA_RESOURCE_DIRS := res
 
 LOCAL_JAVACFLAGS += -g -Xlint
+-include tools/tradefederation/core/error_prone_rules.mk
 
 LOCAL_MODULE := tradefed-contrib