[ios] Add new possible "disabled" state to Firebase metrics

Firebase is only used to record the `first_open` event, so there
is no need to start it if the event was recorded in a previous
execution of the app.

Add a new value to the metrics used to record the state of the
Firebase initialisation.

Bug: 1148441
Change-Id: I3e2bb50f3d2d4458e53e4dc0464e0e19a5504e55
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2534895
Commit-Queue: Sylvain Defresne <sdefresne@chromium.org>
Commit-Queue: Justin Cohen <justincohen@chromium.org>
Auto-Submit: Sylvain Defresne <sdefresne@chromium.org>
Reviewed-by: Justin Cohen <justincohen@chromium.org>
Cr-Commit-Position: refs/heads/master@{#826946}

tools/metrics/histograms/enums.xml

@@ -31901,6 +31901,7 @@ Called by update_feature_policy_enum.py.-->
   <int value="2" label="Enabled, not at First Run"/>
   <int value="3" label="Disabled, Conversion Window expired"/>
   <int value="4" label="Disabled, Legacy User upgraded"/>
+  <int value="5" label="Disabled, First Open already recorded"/>
 </enum>
 
 <enum name="FirstMeaningfulPaintOrdering">