[Firestore] Fix FirestoreIndexValueWriterTest (#7521)

The test was in the wrong package, `firestore`, for the directory it
lives in, `index`.

Author: rlazo

firebase-firestore/src/test/java/com/google/firebase/firestore/index/FirestoreIndexValueWriterTest.java

@@ -12,11 +12,10 @@
 // See the License for the specific language governing permissions and
 // limitations under the License.
 
-package com.google.firebase.firestore;
+package com.google.firebase.firestore.index;
 
-import com.google.firebase.firestore.index.DirectionalIndexByteEncoder;
-import com.google.firebase.firestore.index.FirestoreIndexValueWriter;
-import com.google.firebase.firestore.index.IndexByteEncoder;
+import com.google.firebase.firestore.FieldValue;
+import com.google.firebase.firestore.UserDataReader;
 import com.google.firebase.firestore.model.DatabaseId;
 import com.google.firebase.firestore.model.FieldIndex;
 import com.google.firestore.v1.Value;