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 detached from /apex in the default mount namespace but
still visible in case bootstrap APEXes are needed.

However, there are (mostly, virtual) devices which don't need two mount
namespaces. Those devices don't need to make /bootstrap-apex directory
at all.

Bug: 290148078
Test: atest VendorApexHostTestCases
Test: atest MicrodroidTests
Change-Id: I541cec71d9970b14971d46e01e4808b23590dbed