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