Fix robo test compile error.

Test: m ThemePickerRoboTests
Change-Id: Ie31055d944ad7318903c3e8b977ea97e253b48ec
diff --git a/robolectric_tests/src/com/android/customization/model/theme/ThemeManagerTest.java b/robolectric_tests/src/com/android/customization/model/theme/ThemeManagerTest.java
index bb5df66..50e8647 100644
--- a/robolectric_tests/src/com/android/customization/model/theme/ThemeManagerTest.java
+++ b/robolectric_tests/src/com/android/customization/model/theme/ThemeManagerTest.java
@@ -67,12 +67,12 @@
     @Mock WallpaperSetter mMockWallpaperSetter;
     private OverlayManagerMocks mMockOmHelper;
     private ThemeManager mThemeManager;
-    private Activity mActivity;
+    private FragmentActivity mActivity;
 
     @Before
     public void setUp() {
         MockitoAnnotations.initMocks(this);
-        Activity activity = Robolectric.buildActivity(FragmentActivity.class).get();
+        FragmentActivity activity = Robolectric.buildActivity(FragmentActivity.class).get();
         mActivity = spy(activity);
         mMockOmHelper = new OverlayManagerMocks();
         mMockOmHelper.setUpMock(mMockOm);