Don't use LOCAL_PROGUARD_ENABLED := nosystem

This is the only user of nosystem, and it doesn't seem to serve
any purpose since most of the system flags are then manually
included back again.  Switch to optimization + obfuscation and
let the build system provide the correct flags file.

Test: classes.dex is identical, classes-proguard.jar differs only
      in removed StackMapTable sections (from -dontpreverify?).

Change-Id: I6bae85d10311a982d68cc296a441196ee3115c3d
diff --git a/Android.mk b/Android.mk
index 06ac9e6..37f293b 100644
--- a/Android.mk
+++ b/Android.mk
@@ -64,11 +64,9 @@
     LOCAL_REQUIRED_MODULES:= libframesequence libgiftranscode
 endif
 
-LOCAL_PROGUARD_FLAGS := -ignorewarnings
+LOCAL_PROGUARD_ENABLED := obfuscation optimization
 
-LOCAL_PROGUARD_ENABLED := nosystem
-
-LOCAL_PROGUARD_FLAG_FILES := ../../../build/core/proguard_basic_keeps.flags proguard.flags
+LOCAL_PROGUARD_FLAG_FILES := proguard.flags
 ifeq (eng,$(TARGET_BUILD_VARIANT))
     LOCAL_PROGUARD_FLAG_FILES += proguard-test.flags
 else