Drop the dependency on 'ui' in verify_file().

verify_file() has a dependency on the global variable of 'ui' for
posting the verification progress, which requires the users of
libverifier to provide a UI instance.

This CL adds an optional argument to verify_file() so that it can
post the progress through the provided callback function. As a result,
we can drop the MockUI class in verifier_test.cpp.

Test: recovery_component_test passes.
Test: verify_file() posts progress update when installing an OTA.
Change-Id: I8b87d0f0d99777ea755d33d6dbbe2b6d44243bf1