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