Restrict VM usage to platform_app.

Remove access from untrusted apps and instead grant it to platform_app
(but on user builds as well as debug).

Also restrict any app from creating a vsock_socket; using an already
created one is fine.

Bug: 193373841
Test: Microdroid demo app now gets a denial
Test: Rebuild demo with certifcate: platform, adb install, no denial
Change-Id: I7be011e05244767a42d4c56e26de792db4fe599d
4 files changed