Re-enable SyncService.withEncryptionSync to improve decryption of notifications (#1199)

* Re-enable `SyncService.withEncryptionSync` to improve decryption of notifications.

* Add feature flag

libraries/featureflag/api/src/main/kotlin/io/element/android/libraries/featureflag/api/FeatureFlags.kt

@@ -31,5 +31,11 @@ enum class FeatureFlags(
         title = "Polls",
         description = "Create poll and render poll events in the timeline",
         defaultValue = false,
+    ),
+    UseEncryptionSync(
+        key = "feature.useencryptionsync",
+        title = "Use encryption sync",
+        description = "Use the encryption sync API for decrypting notifications.",
+        defaultValue = true,
     )
 }