Add Labs screen for beta testing of public features (#5465)

* Add Labs screen:

- Make `Feature` have an `isInLabs` boolean to distinguish private feature flags from public ones.
- Have `FeatureFlagsService` provide the list of available flags.
- Display the labs item in the settings screen only if there are available public features.
- Remove public feature toggles from developer options.
- Implement the labs screen with the public features.
- Add a clear cache step to the threads feature toggle
- Update screenshots

---------

Co-authored-by: ElementBot <android@element.io>

libraries/featureflag/test/src/main/java/io/element/android/libraries/featureflag/test/FakeFeature.kt

@@ -0,0 +1,20 @@
+/*
+ * Copyright 2025 New Vector Ltd.
+ *
+ * SPDX-License-Identifier: AGPL-3.0-only OR LicenseRef-Element-Commercial
+ * Please see LICENSE files in the repository root for full details.
+ */
+
+package io.element.android.libraries.featureflag.test
+
+import io.element.android.libraries.core.meta.BuildMeta
+import io.element.android.libraries.featureflag.api.Feature
+
+data class FakeFeature(
+    override val key: String,
+    override val title: String,
+    override val description: String? = null,
+    override val defaultValue: (BuildMeta) -> Boolean = { false },
+    override val isFinished: Boolean = false,
+    override val isInLabs: Boolean = false,
+) : Feature

libraries/featureflag/test/src/main/java/io/element/android/libraries/featureflag/test/FakeFeatureFlagService.kt

@@ -17,6 +17,7 @@ import kotlinx.coroutines.flow.MutableStateFlow
 class FakeFeatureFlagService(
     initialState: Map<String, Boolean> = emptyMap(),
     private val buildMeta: BuildMeta = aBuildMeta(),
+    var providedAvailableFeatures: List<Feature> = emptyList(),
 ) : FeatureFlagService {
     private val enabledFeatures = initialState
         .mapValues { MutableStateFlow(it.value) }
@@ -31,4 +32,8 @@ class FakeFeatureFlagService(
     override fun isFeatureEnabledFlow(feature: Feature): Flow<Boolean> {
         return enabledFeatures.getOrPut(feature.key) { MutableStateFlow(feature.defaultValue(buildMeta)) }
     }
+
+    override fun getAvailableFeatures(): List<Feature> {
+        return providedAvailableFeatures
+    }
 }