Vold: Allow Settings to determine if a switchable pair exists (1/2)

This will eliminate the need for pre-configured device pairs listed
in the build.prop, and display the setting if a pair exists.
This exapnds on commit dc8d157d73fb88ee33b38e724a8aabe7d5e53eee

Change-Id: I5f15a711b5ceabddaf842c252d890545f40b4786