Migrate VirtualMachine API to @SystemApi

Mark all the public members of the existing internal platform API as
@SystemApi (except for some @TestApi methods).

Update system-current.txt & test-current.txt to match.

Modify the demo app and the test APK and dependencies to build against
system / test SDK (respectively), instead of the implementation JAR /
platform APIs. (We still use platform APIs for the native payload code
though.)

Bug: 243512115
Test: atest MicrodroidTests MicrodroidHostTestCases
Test: Install & run demo app
CTS-Coverage-Bug: 244561836
Change-Id: Ibfffbb221f3ac37e03d5cfdbc9208428eeadbb9f
diff --git a/demo/Android.bp b/demo/Android.bp
index 2b234a6..a291ee1 100644
--- a/demo/Android.bp
+++ b/demo/Android.bp
@@ -12,14 +12,9 @@
         "com.android.microdroid.testservice-java",
         "com.google.android.material_material",
     ],
-    libs: [
-        // We need to compile against the .impl library which includes the hidden
-        // APIs. Once the APIs are promoted to @SystemApi we can switch to
-        // framework-virtualization, which contains API stubs.
-        "framework-virtualization.impl",
-    ],
+    sdk_version: "system_current",
     jni_libs: ["MicrodroidTestNativeLib"],
-    platform_apis: true,
+    jni_uses_platform_apis: true,
     use_embedded_native_libs: true,
     v4_signature: true,
     min_sdk_version: "33",