Only allow debuggable Microdroid VMs to mount /vendor

Until the verification of the /vendor partition we restrict the usage of
the feature to only debuggable VMs. If a non-debuggable Microdroid VM
is requested to mount /vendor, first_stage_init will crash and the VM
won't boot.

Bug: 285855436
Test: vm run-microdroid --debug none --vendor test_vendor.img
Change-Id: I9d44ad5c1d971bac1a9173c291ce61b628f2f8e9