Fix incorrect syntax in Proguard flags

When writing a rule matching fields and methods at the same
time, only the name '*' is authorized.

Bug: 34916285

Change-Id: I90cb8273f0915e216940faa1b3d9c6e82296798f

packages/SystemUI/proguard.flags

@@ -38,5 +38,5 @@
 -keep class ** extends android.support.v14.preference.PreferenceFragment
 -keep class com.android.systemui.tuner.*
 -keep class com.android.systemui.plugins.** {
-    public protected **;
+    public protected *;
 }