init: start ueventd in the default mount namespace

Init starts ueventd in the default mount namespace to support loading
firmware from APEXes.

Bug: 155023652
Test: devices boots
      adb$ nsenter -t (pid of ueventd) -m ls /apex
      => shows all APEXes
Change-Id: Ibb8b33a07eb014752275e3bca4541b8b694dc64b
diff --git a/init/mount_namespace.h b/init/mount_namespace.h
index c41a449..d4d6f82 100644
--- a/init/mount_namespace.h
+++ b/init/mount_namespace.h
@@ -16,12 +16,15 @@
 
 #pragma once
 
+#include <android-base/result.h>
+
 namespace android {
 namespace init {
 
+enum MountNamespace { NS_BOOTSTRAP, NS_DEFAULT };
+
 bool SetupMountNamespaces();
-bool SwitchToDefaultMountNamespace();
-bool SwitchToBootstrapMountNamespaceIfNeeded();
+base::Result<void> SwitchToMountNamespaceIfNeeded(MountNamespace target_mount_namespace);
 
 }  // namespace init
 }  // namespace android