Support library enhancements, to ease transition of vold to KM4

Keymaster clients need to see all the available devices and figure out
which they want to use.  This method finds them all and returns them
in a vector sorted from most secure to least, according to a heuristic
defined in Keymaster::VersionResult::operator<

This CL also makes a few other minor improvements to the support
library, providing more information in VersionResult and adding some
more convenience methods in AuthorizationSetBuilder.

Test: Build & boot
Change-Id: I876238ee9ff72573c30d60e1cec665dd610bcde6