Add fsverity digest manifest

fsverity digest manifest stores a map from files to fsverity digests.
The manifest is installed as a serialized protobuf file, to a signed apk
system/etc/security/fsverity/BuildManifest.apk.

Bug: 193113311
Test: build with PRODUCT_SYSTEM_FSVERITY_GENERATE_METADATA := true
Change-Id: I55fc10400206b8ce0d5f198faea08fe3930b362c