Make analytics job ts settings hidden

* There isn't a great reason to allow the UI to edit these meta-data
  fields that denote the last time an analytics job ran.
* The only reason I hesitate to mark them uneditable in the API is that
  they are useful to change in order to influence when the jobs run.
  Mostly for debug purposes or 1-off.

awx/main/conf.py

@@ -869,6 +869,7 @@
     allow_null=True,
     category=_('System'),
     category_slug='system',
+    hidden=True,
 )
 
 register(
@@ -878,6 +879,7 @@
     allow_null=True,
     category=_('System'),
     category_slug='system',
+    hidden=True,
 )
 
 register(