Fix resource problem in PrivateVolumeForgetTest

Somehow Resource IDs are inconsistent between SettingsLib and
SettingsUnitTests. This is a workaround for NotFoundException
in SettingsUnitTests.

Bug: 183342958
Test: atest com.android.settings.deviceinfo.PrivateVolumeForgetTest
Change-Id: If513d5796d4dcd50261d9dd366f6a0def51e0ec4

packages/SettingsLib/MainSwitchPreference/src/com/android/settingslib/widget/MainSwitchBar.java

@@ -63,7 +63,8 @@ public class MainSwitchBar extends LinearLayout implements CompoundButton.OnChec
             int defStyleRes) {
         super(context, attrs, defStyleAttr, defStyleRes);
 
-        LayoutInflater.from(context).inflate(R.layout.main_switch_bar, this);
+        LayoutInflater.from(context).inflate(resourceId(context, "layout", "main_switch_bar"),
+                this);
 
         setFocusable(true);
         setClickable(true);
@@ -255,4 +256,8 @@ public class MainSwitchBar extends LinearLayout implements CompoundButton.OnChec
 
         requestLayout();
     }
+
+    private int resourceId(Context context, String type, String name) {
+        return context.getResources().getIdentifier(name, type, context.getPackageName());
+    }
 }