New API for query trust of a fs-verity certificate

The corresponding service is also added.

The API can be used by a store to know whether their certificate is
trusted on the device. As optimization, they only need to download
.fsv_sig signature file if it will be used.

The API can also be used to gradually switch to stronger key. The store
can query with their certificates in priority order and download the best
signature.

Test: Passed new GTS working in progress
Bug: 142573505
Change-Id: Ic788cd04aeaed35ad62113fe9e7535b8fa63b5ee