Disable the middle pages during development

Set isEnabled to false will prevent these middle pages which under
development get accidentally displayed to users.

During development, please flip isEnabled() of each page to true or
flip SpaEnvironment.IS_DEBUG to true to test these pages.

Bug: 244122804
Test: Manually with Settings
Change-Id: I5c3f835cfa3457b711d224ca37aa5d2b9a248feb

src/com/android/settings/spa/system/SystemMain.kt

@@ -37,6 +37,8 @@ object SystemMainPageProvider : SettingsPageProvider {
     override val name = "SystemMain"
     private val owner = createSettingsPage()
 
+    override fun isEnabled(arguments: Bundle?) = false
+
     @Composable
     override fun Page(arguments: Bundle?) {
         RegularScaffold(title = getTitle(arguments)) {