[fenix] For https://github.com/mozilla-mobile/fenix/issues/25472 - Set `context_id` to "application" lifetime.

This change mitigates it being set only once in FenixApplication and then
being reset after the first ping is sent.
By having "application" lifetime once set in FenixApplication will be available
in all future contile pings for that application run.

app/metrics.yaml

@@ -5095,7 +5095,7 @@ top_sites:
       A UUID that is unjoinable with other browser metrics. This ID will not be
       shared with AdM, only for internal uses. This ID is shared across all
       contextual services features.
-    lifetime: ping
+    lifetime: application
     send_in_pings:
       - topsites-impression
     bugs: