Replace mk_payload with a Java utility

mk_payload was used to create a microdroid's payload metadata blob. It
was too verbose and inefficient to do it via external host tool
(mk_payload).

Bug: 269409253
Test: atest MicrodroidHostTestCases
Change-Id: Ic898e1958be528df2996fe2edb77b4c0b23f04fb
diff --git a/microdroid/payload/Android.bp b/microdroid/payload/Android.bp
index f77c037..da4e09d 100644
--- a/microdroid/payload/Android.bp
+++ b/microdroid/payload/Android.bp
@@ -36,6 +36,17 @@
     ],
 }
 
+java_library_host {
+    name: "microdroid_payload_metadata",
+    srcs: [
+        "src/**/*.java",
+        "metadata.proto",
+    ],
+    proto: {
+        type: "lite",
+    },
+}
+
 cc_binary_host {
     name: "mk_payload",
     srcs: [