Cleaning up build rules to simplify customizing derivative projects

> Using {packageName} instead of hardcoding com.android.launcher3 in AndroidManifest.xml
  for strings which are dependent on packageName
> Adding proguard rule to prevent obfuscating any overridable class
> Making it easier to extend SettingsActivity by overriding the fragment class

Change-Id: I5668c3f33b4cf20ad01d7f54b3d79cc0d268d391

src/com/android/launcher3/MainProcessInitializer.java

@@ -20,14 +20,15 @@ import android.content.Context;
 
 import com.android.launcher3.graphics.IconShapeOverride;
 import com.android.launcher3.logging.FileLog;
+import com.android.launcher3.util.ResourceBasedOverride;
 
 /**
  * Utility class to handle one time initializations of the main process
  */
-public class MainProcessInitializer {
+public class MainProcessInitializer implements ResourceBasedOverride {
 
     public static void initialize(Context context) {
-        Utilities.getOverrideObject(
+        Overrides.getObject(
                 MainProcessInitializer.class, context, R.string.main_process_initializer_class)
                 .init(context);
     }