Use immersive sticky flag when hiding reader UI (closes #2705)

app/src/main/java/eu/kanade/tachiyomi/util/view/WindowExtensions.kt

@@ -15,7 +15,7 @@ fun Window.hideBar() {
             View.SYSTEM_UI_FLAG_LAYOUT_HIDE_NAVIGATION or
             View.SYSTEM_UI_FLAG_FULLSCREEN or
             View.SYSTEM_UI_FLAG_HIDE_NAVIGATION or
-            View.SYSTEM_UI_FLAG_IMMERSIVE
+            View.SYSTEM_UI_FLAG_IMMERSIVE_STICKY
     decorView.systemUiVisibility = uiFlags
 }