ANDROID: fs-verity: Export function to check signatures

Allows a file system to provide its own fs-verity implementation
but still to hook into the signature check and control file from
fs-verity

Bug: 160634504
Bug: 170978993
Test: incfs_test running on this + subsequent changes
Signed-off-by: Paul Lawrence <paullawrence@google.com>
Change-Id: I02020af460d62fa5eb459a083419208e175005e8