Define the protobuf for OTA metadata

Background in http://go/android-partial-updates. For partial update
(e.g. system-only) or devices with mixed build, the current
fingerprint & device name no longer suffice as the precondition to
install the package.

Therefore, we need to additionally include the per-partition build
props into the ota metadata. We also define a protobuf for the metadata
so it can be extended later. The metadata of the legacy format is also
kept for backward compatibility.

Bug: 151088567
Test: unittest pass, generate an OTA and check the result
Change-Id: I716f7da54a393cd340280dbddc3c92b3460f8ef8

tools/releasetools/non_ab_ota.py

@@ -276,7 +276,7 @@ endif;
 
   script.SetProgress(1)
   script.AddToZip(input_zip, output_zip, input_path=OPTIONS.updater_binary)
-  metadata["ota-required-cache"] = str(script.required_cache)
+  metadata.required_cache = script.required_cache
 
   # We haven't written the metadata entry, which will be done in
   # FinalizeMetadata.
@@ -530,7 +530,7 @@ endif;
     script.AddToZip(source_zip, output_zip, input_path=OPTIONS.updater_binary)
   else:
     script.AddToZip(target_zip, output_zip, input_path=OPTIONS.updater_binary)
-  metadata["ota-required-cache"] = str(script.required_cache)
+  metadata.required_cache = script.required_cache
 
   # We haven't written the metadata entry yet, which will be handled in
   # FinalizeMetadata().