Unneeded Firebase services and receivers are not being disabled

[ernestoyaquello]

In the native version of the library, it seems like some Firebase services and receivers get disabled depending on the user configuration:

<service android:enabled="@bool/ll_fcm_push_services_enabled" android:exported="true" android:name="com.localytics.android.FirebaseService">
    <intent-filter>
        <action android:name="com.google.firebase.MESSAGING_EVENT"/>
    </intent-filter>
</service>

In the example shown above, the service com.localytics.android.FirebaseService will only be enabled in the manifest if ll_fcm_push_services_enabled is set to true.

However, this doesn't seem to be the case on the Xamarin version of the library, as the values set in the configuration don't seem to be used in the manifest at all.

[tomcurran]

I've implemented a possible approach to this by switching from using the native library JAR to AAR which then supports manifest merging tomcurran#1 I did it using v6.2.0 of the native library so PR #60 needs to go through first before creating a PR for it