Fix resource problem in PrivateVolumeForgetTest

Somehow Resource IDs are inconsistent between SettingsLib and
SettingsUnitTests. Import SettingsLibMainSwitchPreference to fix.

Bug: 183342958
Test: atest com.android.settings.deviceinfo.PrivateVolumeForgetTest
Change-Id: Id8dfccd520577fc1a8e5177e20e99b52bb57a66a
diff --git a/tests/unit/Android.bp b/tests/unit/Android.bp
index 408f2d3..5582ff4 100644
--- a/tests/unit/Android.bp
+++ b/tests/unit/Android.bp
@@ -35,6 +35,7 @@
         "SettingsLibSettingsSpinner",
         "SettingsLibUsageProgressBarPreference",
         "SettingsLibTwoTargetPreference",
+        "SettingsLibMainSwitchPreference",
     ],
 
     // Include all test java files.