[attestation] Rename test_apk to demo_apk for Vm attestation

A separate test_apk will be added in a subsequent cl for e2e test.

Specifically, the demo_apk calls the real request attestation API
and interacts with RKPD to retrieve the real remotely provisioned
keys; while the test_apk will call the request attestation API
for testing only and will not trigger RKPD, a mock key will be used
in this case.

Test: Run VmAttestationDemoApp manually
Change-Id: I5e02dc071d167156e98088829227e83300899461
diff --git a/vm_payload/Android.bp b/vm_payload/Android.bp
index 286612c..cd7e6f2 100644
--- a/vm_payload/Android.bp
+++ b/vm_payload/Android.bp
@@ -53,7 +53,7 @@
     ],
     visibility: [
         "//packages/modules/Virtualization/compos",
-        "//packages/modules/Virtualization/service_vm/test_apk",
+        "//packages/modules/Virtualization/service_vm:__subpackages__",
     ],
     shared_libs: [
         "libvm_payload#current",