Convert devdb and devdiff to python

Both devdb and devdiff are old and in need of a refresh.

I'd like to have a single module, that supports the following:

create - Create the DB (currently as devdb create)
diff - Diff devices (currently as devdiff)
exposed - Show exposure of devices (currently as devdiff --exposed)

exposed will be the most powerful, and can take advantage of the new userdb module. Examples:

# Running by default should tell me anything a third-party app can use
$ dtf devdb exposed

# We can also say only non-AOSP devices, like sysappdb
$ dtf devdb exposed --new-only

# But let's say I somehow can access the 'inet' group, what now?
$ dtf devdb exposed --new-only --as-user inet

# We should also support many users:
$ dtf devdb exposed --new-only --as-user inet,log,media

jakev / dtfmods-core

Convert devdb and devdiff to python #37