Makes AppDependencyProvider non-instantiatable (#3373)

Task/Issue URL:
https://app.asana.com/0/414235014887631/1208353288582614/f

## Description

This is a follow up to [NetworkProtectionTunnelController
leaking](https://app.asana.com/0/inbox/1203108348814444/1208331423616717/1208348963925370)

We want to make AppDependencyProvider non instantiatable, since it's
really not meant to be.

DuckDuckGo/AppDependencyProvider.swift

@@ -54,7 +54,7 @@ protocol DependencyProvider {
 
 /// Provides dependencies for objects that are not directly instantiated
 /// through `init` call (e.g. ViewControllers created from Storyboards).
-class AppDependencyProvider: DependencyProvider {
+final class AppDependencyProvider: DependencyProvider {
 
     static var shared: DependencyProvider = AppDependencyProvider()
 
@@ -89,12 +89,12 @@ class AppDependencyProvider: DependencyProvider {
     let networkProtectionTunnelController: NetworkProtectionTunnelController
 
     let subscriptionAppGroup = Bundle.main.appGroup(bundle: .subs)
-    
+
     let connectionObserver: ConnectionStatusObserver = ConnectionStatusObserverThroughSession()
     let serverInfoObserver: ConnectionServerInfoObserver = ConnectionServerInfoObserverThroughSession()
     let vpnSettings = VPNSettings(defaults: .networkProtectionGroupDefaults)
 
-    init() {
+    private init() {
         featureFlagger = DefaultFeatureFlagger(internalUserDecider: internalUserDecider,
                                                privacyConfigManager: ContentBlocking.shared.privacyConfigurationManager)
 
@@ -144,4 +144,10 @@ class AppDependencyProvider: DependencyProvider {
         vpnFeatureVisibility = DefaultNetworkProtectionVisibility(userDefaults: .networkProtectionGroupDefaults,
                                                                   accountManager: accountManager)
     }
+
+    /// Only meant to be used for testing.
+    ///
+    static func makeTestingInstance() -> Self {
+        Self.init()
+    }
 }

DuckDuckGoTests/AutoClearSettingsScreenTests.swift

@@ -35,7 +35,7 @@ class AutoClearSettingsScreenTests: XCTestCase {
     override func tearDown() {
         super.tearDown()
 
-        AppDependencyProvider.shared = AppDependencyProvider()
+        AppDependencyProvider.shared = AppDependencyProvider.makeTestingInstance()
     }
 
     func testWhenOpeningSettingsThenClearDataToggleIsSetBasedOnAppSettings() {

DuckDuckGoTests/LargeOmniBarStateTests.swift

@@ -36,7 +36,7 @@ class LargeOmniBarStateTests: XCTestCase {
     override func tearDown() {
         super.tearDown()
 
-        AppDependencyProvider.shared = AppDependencyProvider()
+        AppDependencyProvider.shared = AppDependencyProvider.makeTestingInstance()
     }
 
     func testWhenInHomeEmptyEditingStateWithoutVoiceSearchThenCorrectButtonsAreShown() {

DuckDuckGoTests/MockDependencyProvider.swift

@@ -51,7 +51,7 @@ class MockDependencyProvider: DependencyProvider {
     var vpnSettings: NetworkProtection.VPNSettings
 
     init() {
-        let defaultProvider = AppDependencyProvider()
+        let defaultProvider = AppDependencyProvider.makeTestingInstance()
         appSettings = defaultProvider.appSettings
         variantManager = defaultProvider.variantManager
         featureFlagger = defaultProvider.featureFlagger

DuckDuckGoTests/SmallOmniBarStateTests.swift

@@ -36,7 +36,7 @@ class SmallOmniBarStateTests: XCTestCase {
     override func tearDown() {
         super.tearDown()
 
-        AppDependencyProvider.shared = AppDependencyProvider()
+        AppDependencyProvider.shared = AppDependencyProvider.makeTestingInstance()
     }
 
     func testWhenInHomeEmptyEditingStateWithoutVoiceSearchThenCorrectButtonsAreShown() {

DuckDuckGoTests/Subscription/SubscriptionPagesUseSubscriptionFeatureTests.swift

@@ -169,7 +169,7 @@ final class SubscriptionPagesUseSubscriptionFeatureTests: XCTestCase {
         pixelsFired.removeAll()
         HTTPStubs.removeAllStubs()
 
-        AppDependencyProvider.shared = AppDependencyProvider()
+        AppDependencyProvider.shared = AppDependencyProvider.makeTestingInstance()
 
         subscriptionService = nil
         authService = nil