Add strings for warning when adding a contact is slow.

briar-android/src/main/res/values/strings.xml

@@ -213,6 +213,9 @@
 		<item quantity="one">New contact added.</item>
 		<item quantity="other">%d new contacts added.</item>
 	</plurals>
+	<string name="adding_contact_slow_warning">Adding this contact is taking longer than usual</string>
+	<string name="adding_contact_slow_title">Cannot Connect to Contact</string>
+	<string name="adding_contact_slow_text">Adding this contact is taking longer than usual.\n\nPlease check that your contact has received your link and added you:</string>
 
 	<!-- Introductions -->