[ADD] inject xmlids in an indirect way to avoid warnings

mail_activity/data/ir_model_data.xml

@@ -0,0 +1,20 @@
+<?xml version="1.0" encoding="UTF-8"?>
+<odoo noupdate="1">
+    <!-- inject xmlids so that we can address the records via its
+         mail xmlid as we'd do it in 11.0
+    /-->
+    <record id="xmlid_mail_mt_activities" model="ir.model.data">
+        <field name="model">mail.message.subtype</field>
+        <field name="res_id" ref="mt_activities" />
+        <field name="module">mail</field>
+        <field name="name">mt_activities</field>
+        <field name="noupdate" eval="True" />
+    </record>
+    <record id="xmlid_mail_message_activity_done" model="ir.model.data">
+        <field name="model">ir.ui.view</field>
+        <field name="res_id" ref="message_activity_done" />
+        <field name="module">mail</field>
+        <field name="name">message_activity_done</field>
+        <field name="noupdate" eval="True" />
+    </record>
+</odoo>

mail_activity/data/mail_message_subtype.xml

@@ -1,6 +1,6 @@
 <?xml version="1.0" encoding="UTF-8"?>
 <odoo>
-    <record id="mail.mt_activities" model="mail.message.subtype">
+    <record id="mt_activities" model="mail.message.subtype">
         <field name="name">Activities</field>
         <field name="default" eval="True"/>
         <field name="internal" eval="True"/>

mail_activity/models/__init__.py

@@ -5,3 +5,4 @@ from . import mail_activity_type
 from . import mail_activity
 from . import mail_activity_mixin
 from . import res_partner
+from . import mail_message

mail_activity/views/mail_activity.xml

@@ -161,7 +161,7 @@
         </field>
     </record>
 
-    <template id="mail.message_activity_done">
+    <template id="message_activity_done">
 <div>
     <p>
         <strong><span t-attf-class="fa #{activity.activity_type_id.icon}"/></strong> <span t-field="activity.activity_type_id.name"/> done