[fenix] update isFirstRun to be manually set and set experiments locally on first run

app/src/main/java/org/mozilla/fenix/experiments/NimbusSetup.kt

@@ -101,7 +101,8 @@ fun createNimbus(context: Context, url: String?): NimbusApi {
                 globalUserParticipation = enabled
             }
 
-            if (url.isNullOrBlank()) {
+            if (context.settings().isFirstRun || url.isNullOrBlank()) {
+                context.settings().isFirstRun = false
                 setExperimentsLocally(R.raw.initial_experiments)
             }
 

app/src/main/java/org/mozilla/fenix/utils/Settings.kt

@@ -347,18 +347,10 @@ class Settings(private val appContext: Context) : PreferencesHolder {
         default = false
     )
 
-    val isFirstRun: Boolean =
-        if (!preferences.contains(appContext.getPreferenceKey(R.string.pref_key_is_first_run))) {
-            preferences.edit()
-                .putBoolean(
+    val isFirstRun: Boolean by booleanPreference(
         appContext.getPreferenceKey(R.string.pref_key_is_first_run),
-                    false
+        default = true
     )
-                .apply()
-            true
-        } else {
-            false
-        }
 
     /**
      * Indicates the last time when the user was interacting with the [BrowserFragment],