[fenix] For https://github.com/mozilla-mobile/fenix/issues/25213: Add tabs prioritization experiment

.experimenter.yaml

@@ -7,6 +7,14 @@ default-browser-message:
     message-location:
       type: string
       description: Where is the message to be put.
+engine-settings:
+  description: Contains a set of settings for controlling the web engine configurations.
+  hasExposure: true
+  exposureDescription: ""
+  variables:
+    tabs-prioritization-enabled:
+      type: boolean
+      description: "If true, enables tabs prioritization feature."
 homescreen:
   description: The homescreen that the user goes to when they press home or new tab.
   hasExposure: true

app/src/main/java/org/mozilla/fenix/components/Core.kt

@@ -88,6 +88,7 @@ import org.mozilla.fenix.historymetadata.DefaultHistoryMetadataService
 import org.mozilla.fenix.historymetadata.HistoryMetadataMiddleware
 import org.mozilla.fenix.historymetadata.HistoryMetadataService
 import org.mozilla.fenix.media.MediaSessionService
+import org.mozilla.fenix.nimbus.FxNimbus
 import org.mozilla.fenix.perf.StrictModeManager
 import org.mozilla.fenix.perf.lazyMonitored
 import org.mozilla.fenix.settings.SupportUtils
@@ -224,6 +225,8 @@ class Core(
      * The [BrowserStore] holds the global [BrowserState].
      */
     val store by lazyMonitored {
+        val tabsPrioritizationEnable =
+            FxNimbus.features.engineSettings.value().tabsPrioritizationEnabled
         val middlewareList =
             mutableListOf(
                 LastAccessMiddleware(),
@@ -244,9 +247,12 @@ class Core(
                 AdsTelemetryMiddleware(adsTelemetry),
                 LastMediaAccessMiddleware(),
                 HistoryMetadataMiddleware(historyMetadataService),
-                SearchTermTabGroupMiddleware(),
+                SearchTermTabGroupMiddleware()
-                SessionPrioritizationMiddleware()
+            ) + if (tabsPrioritizationEnable) {
-            )
+                listOf(SessionPrioritizationMiddleware())
+            } else {
+                emptyList()
+            }
 
         BrowserStore(
             initialState = BrowserState(

nimbus.fml.yaml

@@ -203,6 +203,14 @@ features:
         value:
           enabled: true
 
+  engine-settings:
+    description: Contains a set of settings for controlling the web engine configurations.
+    variables:
+      tabs-prioritization-enabled:
+        description: If true, enables tabs prioritization feature.
+        type: Boolean
+        default: true
+
 types:
   objects:
     MessageData: