For #17447: add intentional delay to cold start up in debug and nightly.

I validated:
- that the log statement appeared in Nightly but not in Beta.
- that the local runtimes of our perftest increased (the median diff is 124ms)

app/src/main/java/org/mozilla/fenix/FeatureFlags.kt

@@ -39,6 +39,13 @@ object FeatureFlags {
     // users are still experiencing crashes.
     const val nimbusExperiments = false
 
+    /**
+     * Enables an intentional regression to validate perftest alerting. See
+     * https://github.com/mozilla-mobile/fenix/issues/17447 for details. This
+     * is expected to be removed within several days.
+     */
+    val intentionalRegressionToValidatePerfTestAlerting = Config.channel.isNightlyOrDebug
+
     /**
      * Enables the new MediaSession API.
      */

app/src/main/java/org/mozilla/fenix/FenixApplication.kt

@@ -119,6 +119,13 @@ open class FenixApplication : LocaleAwareApplication(), Provider {
 
     @CallSuper
     open fun setupInMainProcessOnly() {
+        // See feature flags kdoc for details.
+        if (FeatureFlags.intentionalRegressionToValidatePerfTestAlerting) {
+            logger.info("Intentional thread sleep. See #17447")
+            @Suppress("MagicNumber") // it's fine for a quick patch.
+            Thread.sleep(100)
+        }
+
         run {
             // Attention: Do not invoke any code from a-s in this scope.
             val megazordSetup = setupMegazord()