Rename payload metadata partition

/dev/block/by-name/metadata is used by others as well.

Renamed it as payload-metadata.

Bug: 191097666
Test: MicrodroidHostTestCases
Change-Id: I873ee2acbe38772e59c96b3904023ac0d1573621
diff --git a/microdroid/payload/mk_payload.cc b/microdroid/payload/mk_payload.cc
index c31dcff..b27683c 100644
--- a/microdroid/payload/mk_payload.cc
+++ b/microdroid/payload/mk_payload.cc
@@ -302,7 +302,7 @@
 
     // put metadata at the first partition
     partitions.push_back(MultipleImagePartition{
-            .label = "metadata",
+            .label = "payload-metadata",
             .image_file_paths = {metadata_file},
             .type = kLinuxFilesystem,
             .read_only = true,