microdroid: add "apk" to payload

For now apk's path should be specified in the config. But in the future
it will be retrieved from apk package name. APK file's idsig is not
passed yet.

Bug: n/a
Test: MicrodroidHostTestCases
Change-Id: I6916c657b29b72a809a69f8be41c2398314e0178
diff --git a/microdroid/signature/microdroid_signature.proto b/microdroid/signature/microdroid_signature.proto
index 8335ff5..8816aa8 100644
--- a/microdroid/signature/microdroid_signature.proto
+++ b/microdroid/signature/microdroid_signature.proto
@@ -25,6 +25,8 @@
   // Lists the signature information of the payload apexes.
   // The payload apexes are mapped to the partitions following the signature partition.
   repeated ApexSignature apexes = 2;
+
+  ApkSignature apk = 3;
 }
 
 message ApexSignature {
@@ -44,3 +46,12 @@
   // When specified, the root digest of the apex should match with it.
   string rootDigest = 4;
 }
+
+message ApkSignature {
+  // Required.
+  // The name of APK.
+  string name = 1;
+
+  string payload_partition_name = 2;
+  string idsig_partition_name = 3;
+}
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