Enable Proguard for eng build too.

With this change, all build variants are now with Proguard enabled,
unless you explicitly set DISABLE_PROGUARD to true.

Change-Id: I0f9b566b5ab3c3d961ffd6ab696e573bc59553b0
diff --git a/core/package.mk b/core/package.mk
index 2012351..ff202cb 100644
--- a/core/package.mk
+++ b/core/package.mk
@@ -130,8 +130,7 @@
 
 LOCAL_PROGUARD_ENABLED:=$(strip $(LOCAL_PROGUARD_ENABLED))
 ifndef LOCAL_PROGUARD_ENABLED
-ifneq ($(filter user userdebug, $(TARGET_BUILD_VARIANT)),)
-    # turn on Proguard by default for user & userdebug build
+ifneq ($(DISABLE_PROGUARD),true)
     LOCAL_PROGUARD_ENABLED :=full
 endif
 endif