Add new LOCAL_PROGUARD_ENABLED option "nosystem"

With which we don't include proguard flags defined by the build system.
Bug: 7990659

Change-Id: I78384d2fbaccdd714d646e1b648417195bbd8dde
diff --git a/core/clear_vars.mk b/core/clear_vars.mk
index 91b5df6..e56180d 100644
--- a/core/clear_vars.mk
+++ b/core/clear_vars.mk
@@ -100,7 +100,7 @@
 LOCAL_SDK_RES_VERSION:=
 LOCAL_NDK_STL_VARIANT:=
 LOCAL_EMMA_INSTRUMENT:=
-LOCAL_PROGUARD_ENABLED:= # '',optonly,full,custom,disabled
+LOCAL_PROGUARD_ENABLED:= # '',full,custom,nosystem,disabled
 LOCAL_PROGUARD_FLAGS:=
 LOCAL_PROGUARD_FLAG_FILES:=
 LOCAL_EMMA_COVERAGE_FILTER:=