For 4780: update HomeActivity to maintain private mode in memory

As an added bonus, this makes the temporal coupling between `setPrivateModeIfNecessary` and `setupThemeAndBrowsingMode` explicit.  They previously would have broken if called in reverse order, now it will fail to compile.

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

@@ -85,10 +85,10 @@ open class HomeActivity : AppCompatActivity() {
     final override fun onCreate(savedInstanceState: Bundle?) {
         super.onCreate(savedInstanceState)
 
-        setPrivateModeIfNecessary()
+        val mode = setPrivateModeIfNecessary()
 
         components.publicSuffixList.prefetch()
-        setupThemeAndBrowsingMode()
+        setupThemeAndBrowsingMode(mode)
 
         setContentView(R.layout.activity_home)
 
@@ -191,18 +191,20 @@ open class HomeActivity : AppCompatActivity() {
      * External sources such as 3rd party links and shortcuts use this function to enter
      * private mode directly before the content view is created.
      */
-    private fun setPrivateModeIfNecessary() {
+    private fun setPrivateModeIfNecessary(): BrowsingMode {
         intent?.toSafeIntent()?.let {
             if (it.hasExtra(PRIVATE_BROWSING_MODE)) {
                 val startPrivateMode = it.getBooleanExtra(PRIVATE_BROWSING_MODE, false)
-                settings().usePrivateMode = startPrivateMode
                 intent.removeExtra(PRIVATE_BROWSING_MODE)
+
+                return BrowsingMode.fromBoolean(isPrivate = startPrivateMode)
             }
         }
+        return BrowsingMode.Normal
     }
 
-    private fun setupThemeAndBrowsingMode() {
-        browsingModeManager = createBrowsingModeManager()
+    private fun setupThemeAndBrowsingMode(mode: BrowsingMode) {
+        browsingModeManager = createBrowsingModeManager(mode)
         themeManager = createThemeManager()
         themeManager.setActivityTheme(this)
         themeManager.applyStatusBarTheme(this)
@@ -317,9 +319,9 @@ open class HomeActivity : AppCompatActivity() {
         }
     }
 
-    protected open fun createBrowsingModeManager(): BrowsingModeManager {
-        return DefaultBrowsingModeManager(settings()) { mode ->
-            themeManager.currentTheme = mode
+    protected open fun createBrowsingModeManager(initialMode: BrowsingMode): BrowsingModeManager {
+        return DefaultBrowsingModeManager(initialMode) { newMode ->
+            themeManager.currentTheme = newMode
         }
     }
 

app/src/main/java/org/mozilla/fenix/browser/browsingmode/BrowsingModeManager.kt

@@ -31,14 +31,18 @@ interface BrowsingModeManager {
     var mode: BrowsingMode
 }
 
+/**
+ * Wraps a [BrowsingMode] and executes a callback whenever [mode] is updated.
+ */
 class DefaultBrowsingModeManager(
-    private val settings: Settings,
+    private var _mode: BrowsingMode,
     private val modeDidChange: (BrowsingMode) -> Unit
 ) : BrowsingModeManager {
+
     override var mode: BrowsingMode
-        get() = BrowsingMode.fromBoolean(settings.usePrivateMode)
+        get() = _mode
         set(value) {
-            settings.usePrivateMode = value.isPrivate
+            _mode = value
             modeDidChange(value)
         }
 }

app/src/main/java/org/mozilla/fenix/customtabs/ExternalAppBrowserActivity.kt

@@ -11,6 +11,7 @@ import mozilla.components.support.utils.SafeIntent
 import org.mozilla.fenix.BrowserDirection
 import org.mozilla.fenix.HomeActivity
 import org.mozilla.fenix.NavGraphDirections
+import org.mozilla.fenix.browser.browsingmode.BrowsingMode
 import org.mozilla.fenix.browser.browsingmode.CustomTabBrowsingModeManager
 import org.mozilla.fenix.components.metrics.Event
 import org.mozilla.fenix.ext.components
@@ -42,7 +43,7 @@ open class ExternalAppBrowserActivity : HomeActivity() {
         )
     }
 
-    final override fun createBrowsingModeManager() =
+    final override fun createBrowsingModeManager(initialMode: BrowsingMode) =
         CustomTabBrowsingModeManager()
 
     final override fun createThemeManager() = CustomTabThemeManager()