Port flaky test fix from web.

Port of firebase/firebase-js-sdk#1511

Author: Michael Lehenbauer

firebase-firestore/src/androidTest/java/com/google/firebase/firestore/ServerTimestampTest.java

@@ -215,7 +215,8 @@ public void testServerTimestampsCanRetainPreviousValueThroughConsecutiveUpdates(
     DocumentSnapshot localSnapshot = accumulator.awaitLocalEvent();
     assertEquals(42L, localSnapshot.get("a", ServerTimestampBehavior.PREVIOUS));
 
-    docRef.update("a", FieldValue.serverTimestamp());
+    // include b=1 to ensure there's a change resulting in a new snapshot.
+    docRef.update("a", FieldValue.serverTimestamp(), "b", 1);
     localSnapshot = accumulator.awaitLocalEvent();
     assertEquals(42L, localSnapshot.get("a", ServerTimestampBehavior.PREVIOUS));