Fix robolectric test failure

Default UserManager caches instance across entire test suite, so 
test becomes flakey depending on the order they are run. Using a 
a ShadowUserManager without the cache ensures each test class gets
a UserManager with clean state.

Change-Id: Ia54f6a3259859add5a1e5d0101829497fb985ab1
Fix: 38393235
Test: make RunSettingsRoboTests
diff --git a/tests/robotests/src/com/android/settings/system/SystemDashboardFragmentTest.java b/tests/robotests/src/com/android/settings/system/SystemDashboardFragmentTest.java
index 65b39e6..483dee7 100644
--- a/tests/robotests/src/com/android/settings/system/SystemDashboardFragmentTest.java
+++ b/tests/robotests/src/com/android/settings/system/SystemDashboardFragmentTest.java
@@ -17,11 +17,13 @@
 package com.android.settings.system;
 
 import android.content.Context;
-
 import android.os.UserManager;
+
 import com.android.settings.SettingsRobolectricTestRunner;
 import com.android.settings.TestConfig;
 import com.android.settings.testutils.XmlTestUtils;
+import com.android.settings.testutils.shadow.ShadowUserManager;
+
 import org.junit.Test;
 import org.junit.runner.RunWith;
 import org.robolectric.RuntimeEnvironment;
@@ -36,7 +38,10 @@
 import static org.mockito.Mockito.when;
 
 @RunWith(SettingsRobolectricTestRunner.class)
-@Config(manifest = TestConfig.MANIFEST_PATH, sdk = TestConfig.SDK_VERSION)
+@Config(manifest = TestConfig.MANIFEST_PATH, sdk = TestConfig.SDK_VERSION,
+        shadows = {
+                ShadowUserManager.class
+        })
 public class SystemDashboardFragmentTest {
 
     @Test