refactor(audience-sample): use EventPropertyKeys for typed-event field lookups in live-fire tests

SampleAppLiveFireTests filled the Resource, Purchase, and
MilestoneReached typed-event forms by looking up TextFields with
inline property-name strings ("currency", "amount", "value", "name").
The SDK already centralises these as EventPropertyKeys.Currency /
.Amount / .Value / .Name, made visible to the sample-app tests
through the existing InternalsVisibleTo grant.

Reference EventPropertyKeys.X from the five lookup sites instead of
re-typing the wire-format strings.

Discovered during the sample-app demo property-key centralisation
pass; kept separate because these keys belong to the SDK's typed-event
surface, not the sample-app demo catalogue.

Per the user's "everything random goes in a constant" stance.

Follow-up to SDK-272 (centralisation of duplicated literals).

Author: ImmutableJeffrey

examples/audience/Assets/SampleApp/Tests/Runtime/SampleAppLiveFireTests.cs

@@ -152,8 +152,8 @@ public IEnumerator TypedEvent_Resource_FlushReportsOk()
         {
             yield return DriveTypedEventAndFlush(SampleAppUi.Buttons.TypedEvent(EventNames.Resource), root =>
             {
-                root.Q<TextField>(SampleAppUi.TypedEventField(EventNames.Resource, "currency")).value = "GOLD";
-                root.Q<TextField>(SampleAppUi.TypedEventField(EventNames.Resource, "amount")).value = "100";
+                root.Q<TextField>(SampleAppUi.TypedEventField(EventNames.Resource, EventPropertyKeys.Currency)).value = "GOLD";
+                root.Q<TextField>(SampleAppUi.TypedEventField(EventNames.Resource, EventPropertyKeys.Amount)).value = "100";
             });
         }
 
@@ -162,8 +162,8 @@ public IEnumerator TypedEvent_Purchase_FlushReportsOk()
         {
             yield return DriveTypedEventAndFlush(SampleAppUi.Buttons.TypedEvent(EventNames.Purchase), root =>
             {
-                root.Q<TextField>(SampleAppUi.TypedEventField(EventNames.Purchase, "currency")).value = "USD";
-                root.Q<TextField>(SampleAppUi.TypedEventField(EventNames.Purchase, "value")).value = "9.99";
+                root.Q<TextField>(SampleAppUi.TypedEventField(EventNames.Purchase, EventPropertyKeys.Currency)).value = "USD";
+                root.Q<TextField>(SampleAppUi.TypedEventField(EventNames.Purchase, EventPropertyKeys.Value)).value = "9.99";
             });
         }
 
@@ -172,7 +172,7 @@ public IEnumerator TypedEvent_MilestoneReached_FlushReportsOk()
         {
             yield return DriveTypedEventAndFlush(SampleAppUi.Buttons.TypedEvent(EventNames.MilestoneReached), root =>
             {
-                root.Q<TextField>(SampleAppUi.TypedEventField(EventNames.MilestoneReached, "name")).value = "il2cpp_smoke";
+                root.Q<TextField>(SampleAppUi.TypedEventField(EventNames.MilestoneReached, EventPropertyKeys.Name)).value = "il2cpp_smoke";
             });
         }