Reland: Pass apex_info.pb file to delta_generator

target_files contain META/apex_info.pb, which contains metadata about
compressed apexes. Extract this file from target_file.zip, and pass it
to delta_generator. delta_generator will then copy these data to
update_metadata

Test: generate an OTA, make sure a device running an older build can
install the OTA
Bug: 172911822

Change-Id: If0e185a32262a849d533c3316ffdf205cb6628b6
diff --git a/update_metadata.proto b/update_metadata.proto
index ffc78b6..a1f093c 100644
--- a/update_metadata.proto
+++ b/update_metadata.proto
@@ -365,6 +365,12 @@
   optional int64 decompressed_size = 4;
 }
 
+// Definition has been duplicated from
+// $ANDROID_BUILD_TOP/build/tools/releasetools/ota_metadata.proto. Keep in sync.
+message ApexMetadata {
+  repeated ApexInfo apex_info = 1;
+}
+
 message DeltaArchiveManifest {
   // Only present in major version = 1. List of install operations for the
   // kernel and rootfs partitions. For major version = 2 see the |partitions|