Make exporting tombstone (out of VM) configurable

VM owner must specify export_tombstones value in assets/vm_config in
payload apk to enable/disable exporting the tombstones out of VM. This
is also dm-verity protected so only the owner of the VM can make the
change.  There is no default value for this for now. Virtualization
service will fail at load_config() throw binder_exception to client.

Test: atest . (all tests in /packages/modules/Virtualization)
Bug: 227443903

Change-Id: I82325a9ffc57d4d4825d51d9d41ebb767e6c1f6f
diff --git a/demo/assets/vm_config.json b/demo/assets/vm_config.json
index b814394..da420ff 100644
--- a/demo/assets/vm_config.json
+++ b/demo/assets/vm_config.json
@@ -9,5 +9,6 @@
       "hello",
       "microdroid"
     ]
-  }
+  },
+  "export_tombstones": true
 }