The problem is (as the error message shows) the same permission is available in another manifest (firebase manifest). So you can remove it from your manifest to solve the problem (easy approach). You can also override it in your manifest like below:
<permission android:name="<your-package-name>.permission.C2D_MESSAGE"
android:protectionLevel="signature"
tools:replace="android:name"/>
For using tools you need to add the xmlns:tools="http://schemas.android.com/tools"
line in the manifest tag as below:
<manifest xmlns:android="http://schemas.android.com/apk/res/android"
xmlns:tools="http://schemas.android.com/tools"
android:allowBackup="false"
android:label="@string/app_name"/>