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/metadata.proto b/microdroid/payload/metadata.proto
index c74c23b..6b999af 100644
--- a/microdroid/payload/metadata.proto
+++ b/microdroid/payload/metadata.proto
@@ -18,6 +18,9 @@
 
 package android.microdroid;
 
+option java_package = "com.android.virt";
+option java_outer_classname = "PayloadMetadataProtos";
+
 // Metadata is the body of the "metadata" partition
 message Metadata {
   uint32 version = 1;