microdroid_manager: initial impl

It is started by init in microdroid and executes a command specified in
a VM payload config.

Bug: 189301496
Test: MicrodroidHostTestCases
  (in a microdroid, run /system/bin/microdroid_manager manulally)
Change-Id: I85c7e370d4a0dcf58b4aafbe6e9fba73e69c2a44
diff --git a/microdroid/signature/microdroid_signature.proto b/microdroid/signature/microdroid_signature.proto
index 8816aa8..6ae3756 100644
--- a/microdroid/signature/microdroid_signature.proto
+++ b/microdroid/signature/microdroid_signature.proto
@@ -27,6 +27,8 @@
   repeated ApexSignature apexes = 2;
 
   ApkSignature apk = 3;
+
+  string payload_config_path = 4;
 }
 
 message ApexSignature {
@@ -54,4 +56,4 @@
 
   string payload_partition_name = 2;
   string idsig_partition_name = 3;
-}
\ No newline at end of file
+}