Use stub APK hiding method for Android 5.0+

At the same time, disable app hiding on devices lower than 5.0
to simplify the logic in the app. By doing so, a hidden app always
implies running as stub.

app/src/main/java/com/topjohnwu/magisk/ui/settings/SettingsViewModel.kt

@@ -59,8 +59,12 @@ class SettingsViewModel(
         ))
         if (Info.env.isActive) {
             list.add(ClearRepoCache)
-            if (Const.USER_ID == 0 && Info.isConnected.get())
-                list.add(if (hidden) Restore else Hide)
+            if (Build.VERSION.SDK_INT >= 21 && Const.USER_ID == 0) {
+                if (hidden)
+                    list.add(Restore)
+                else if (Info.isConnected.get())
+                    list.add(Hide)
+            }
         }
 
         // Magisk