Declare `SyncNotificationsService` only in `prod` sourceSet

And move Firebase Messaging dependency to the appropriate configuration.

Relates to #360

```
nowinandroid/app/src/main/AndroidManifest.xml Warning:
        meta-data#com.google.samples.apps.nowinandroid.sync.initializers.SyncInitializer was tagged at AndroidManifest.xml:0 to remove other declarations but no other declaration present
```

sync/work/build.gradle.kts

@@ -35,10 +35,11 @@ dependencies {
     implementation(libs.androidx.lifecycle.livedata.ktx)
     implementation(libs.androidx.tracing.ktx)
     implementation(libs.androidx.work.ktx)
-    implementation(libs.firebase.cloud.messaging)
     implementation(libs.hilt.ext.work)
     implementation(libs.kotlinx.coroutines.android)
 
+    prodImplementation(libs.firebase.cloud.messaging)
+
     kapt(libs.hilt.ext.compiler)
 
     testImplementation(project(":core:testing"))

sync/work/src/demo/AndroidManifest.xml

@@ -1,27 +0,0 @@
-<?xml version="1.0" encoding="utf-8"?>
-<!--
-     Copyright 2022 The Android Open Source Project
-
-     Licensed under the Apache License, Version 2.0 (the "License");
-     you may not use this file except in compliance with the License.
-     You may obtain a copy of the License at
-
-          http://www.apache.org/licenses/LICENSE-2.0
-
-     Unless required by applicable law or agreed to in writing, software
-     distributed under the License is distributed on an "AS IS" BASIS,
-     WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
-     See the License for the specific language governing permissions and
-     limitations under the License.
--->
-<manifest xmlns:android="http://schemas.android.com/apk/res/android"
-    xmlns:tools="http://schemas.android.com/tools">
-
-    <application>
-        <service
-            android:name=".services.SyncNotificationsService"
-            android:exported="false"
-            tools:node="remove" />
-    </application>
-
-</manifest>