Set apexd.payload_metadata build property

This is currently hard-coded in apexd but will be
changed to a build property so it is configurable.

Bug: 191097666
Test: Presubmit
Change-Id: I68125f7ec9b08a7b7a761cd084327d3a2359be4e
diff --git a/microdroid/build.prop b/microdroid/build.prop
index ada945d..4ae50c0 100644
--- a/microdroid/build.prop
+++ b/microdroid/build.prop
@@ -6,3 +6,6 @@
 # TODO(b/189164487): support build related properties
 ro.build.version.release=11
 ro.build.version.security_patch=2021-07-05
+
+# Payload metadata partition
+apexd.payload_metadata.path=/dev/block/by-name/payload-metadata