Use /bootstrap-apex for bootstrap APEXes

This new directory is bind-mounted to /apex in the bootstrap mount
namespace so that apexd-bootstrap mounts bootstrap APEXes there via
/apex.

The directory is shared between two mount namespaces, hence visible
in the default mount namespace.

Bug: 290148078
Test: VendorApexHostTestCases
Change-Id: I841480e41be8def5a4c6a4aa874c4e21465a71d3
diff --git a/init/mount_namespace.h b/init/mount_namespace.h
index 5e3dab2..43c5476 100644
--- a/init/mount_namespace.h
+++ b/init/mount_namespace.h
@@ -24,9 +24,12 @@
 enum MountNamespace { NS_BOOTSTRAP, NS_DEFAULT };
 
 bool SetupMountNamespaces();
+
 base::Result<void> SwitchToMountNamespaceIfNeeded(MountNamespace target_mount_namespace);
 
 base::Result<MountNamespace> GetCurrentMountNamespace();
 
+bool NeedsTwoMountNamespaces();
+
 }  // namespace init
 }  // namespace android