Fix NPE of LaunchAccessibilityActivityPreferenceFragmentTest.

ComponentName.unflattenFromString() return null if the string was
incorrect.

Bug: 218975513
Test: make -j64 RunSettingsRoboTests ROBOTEST_FILTER=LaunchAccessibilityActivityPreferenceFragmentTest
Change-Id: I21a7bdb14aba6b23e74b9569d0a1a5f74e0f0106

tests/robotests/src/com/android/settings/accessibility/LaunchAccessibilityActivityPreferenceFragmentTest.java

@@ -83,9 +83,8 @@ public class LaunchAccessibilityActivityPreferenceFragmentTest {
     }
 
     @Test
-    public void getTileName_noTileServiceAssigned_noMatchString() {
-        final CharSequence tileName = mFragment.getTileName();
-        assertThat(tileName.toString()).isEqualTo("");
+    public void getTileName_noTileServiceAssigned_returnNull() {
+        assertThat(mFragment.getTileName()).isNull();
     }
 
     @Test