[pm][expand] Write more metadata files

When expanding an archived package, also generate and write the merkle
root of the meta.far (meta.far.merkle) and the blob metadata file
(blobs.json) so the prebuilt_package template/script don't need to parse
the meta/contents file to generate a correct blobs.json file.

Test: build topaz, verify generated outputs of chromium
Change-Id: I6bd01c01fe08bb2169a16b640f6cc539a08bdc04
1 file changed