Fix SharedPrefsBackupHelper so it doesn't hard code the paths to the files.

This took quite a bit of refactoring.

core/java/android/content/Context.java

@@ -254,11 +254,19 @@ public abstract class Context {
      * <p>Note: this is not generally useful for applications, since they should
      * not be directly accessing the file system.
      *
-     *
      * @return String Path to the code and assets.
      */
     public abstract String getPackageCodePath();
 
+    /**
+     * {@hide}
+     * Return the full path to the shared prefs file for the given prefs group name.
+     *
+     * <p>Note: this is not generally useful for applications, since they should
+     * not be directly accessing the file system.
+     */
+    public abstract File getSharedPrefsFile(String name);
+
     /**
      * Retrieve and hold the contents of the preferences file 'name', returning
      * a SharedPreferences through which you can retrieve and modify its

core/java/android/content/ContextWrapper.java

@@ -129,6 +129,11 @@ public class ContextWrapper extends Context {
         return mBase.getPackageCodePath();
     }
 
+    @Override
+    public File getSharedPrefsFile(String name) {
+        return mBase.getSharedPrefsFile(name);
+    }
+
     @Override
     public SharedPreferences getSharedPreferences(String name, int mode) {
         return mBase.getSharedPreferences(name, mode);