Disable Proguard for cts tests.

Previously the build system would include
build/core/proguard_tests.flags, which virutally disables Proguard,
if an app links against android.test.runner.
Now you have to disable it explicitly.

Bug: 10307372
Change-Id: I78ea75ef4770f4a4b761b9b9996e745f417646cc
diff --git a/build/test_package.mk b/build/test_package.mk
index aa9a4dd..46718a8 100644
--- a/build/test_package.mk
+++ b/build/test_package.mk
@@ -21,6 +21,7 @@
 
 # Disable by default so "m cts" will work in emulator builds
 LOCAL_DEX_PREOPT := false
+LOCAL_PROGUARD_ENABLED := disabled
 
 include $(BUILD_PACKAGE)