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/tests/helper/Android.bp b/tests/helper/Android.bp
index 7473dab..61c5dcd 100644
--- a/tests/helper/Android.bp
+++ b/tests/helper/Android.bp
@@ -6,9 +6,7 @@
     name: "MicrodroidTestHelper",
     srcs: ["src/java/com/android/microdroid/test/common/*.java"],
     host_supported: true,
-    libs: [
-        "framework-annotations-lib",
-    ],
+    sdk_version: "system_current",
 }
 
 java_library_static {
@@ -20,8 +18,5 @@
         "MicrodroidTestHelper",
         "truth-prebuilt",
     ],
-    // 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.
-    libs: ["framework-virtualization.impl"],
+    sdk_version: "system_current",
 }