Emit apex_info to target_files META/apex_info.pb

Test: mm -j
Bug: 172911822

Change-Id: I5a70b75dbe1cd546c66beaa8e4e3a63c3987461e
diff --git a/tools/releasetools/ota_metadata.proto b/tools/releasetools/ota_metadata.proto
index 1277685..5da8b84 100644
--- a/tools/releasetools/ota_metadata.proto
+++ b/tools/releasetools/ota_metadata.proto
@@ -16,8 +16,8 @@
 
 // If you change this file,
 // Please update ota_metadata_pb2.py by executing
-// protoc ota_metadata.proto --python_out $ANDROID_BUILD_TOP/build/tools/releasetools
-
+// protoc ota_metadata.proto --python_out
+// $ANDROID_BUILD_TOP/build/tools/releasetools
 
 syntax = "proto3";
 
@@ -72,6 +72,12 @@
   int64 decompressed_size = 4;
 }
 
+// Just a container to hold repeated apex_info, so that we can easily serialize
+// a list of apex_info to string.
+message ApexMetadata {
+  repeated ApexInfo apex_info = 1;
+}
+
 // The metadata of an OTA package. It contains the information of the package
 // and prerequisite to install the update correctly.
 message OtaMetadata {