Allow missing and extra translations

The translation threshold is 80%, so we should allow missing translations, like we do for Briar.

mailbox-android/build.gradle

@@ -64,6 +64,10 @@ android {
         exclude 'META-INF/licenses/*'
         exclude '**/attach_hotspot_windows.dll'
     }
+    lintOptions {
+        warning 'MissingTranslation'
+        warning 'ExtraTranslation'
+    }
 }
 
 configurations {