Add mechanism to grant USB exceptions.

This mechanism allows VID/PID pairs to be registered that will always
appear to have succeeded in having permission granted to them. The
permission broker will take no action in finding the underlying device.
Instead, it is assumed that other system mechanisms will grant access
(udev rules, etc.)

BUG=chromium-os:33830
TEST=manual

Change-Id: I3324e772a8e3122962e6b9fba3d2e27d3e877d9e
2 files changed