Use FakeTimer in test to avoid flakiness.

**Root cause**
ScreenFlashNotificationColorDialogFragment creates Timer which spawns
unmanageable threads for tests. Also, the test itself uses Thread.sleep
which makes it non-destermistic.

Bug: 279082331
Test: atest ScreenFlashNotificationColorDialogFragmentTest --iteration
20

Change-Id: Id49c8d402a0578f8297ca12fe49da304c7a988d8

src/com/android/settings/accessibility/ScreenFlashNotificationColorDialogFragment.java

@@ -119,7 +119,7 @@ public class ScreenFlashNotificationColorDialogFragment extends DialogFragment i
         synchronized (this) {
             if (mTimer != null) mTimer.cancel();
 
-            mTimer = new Timer();
+            mTimer = createTimer();
             if (mIsPreview) {
                 mTimer.schedule(getStopTask(), 0);
                 startDelay = BETWEEN_STOP_AND_START_DELAY_MS;
@@ -176,4 +176,8 @@ public class ScreenFlashNotificationColorDialogFragment extends DialogFragment i
         getContext().sendBroadcast(stopIntent);
         mIsPreview = false;
     }
+
+    Timer createTimer() {
+        return new Timer();
+    }
 }