Add a service to track devices selected by the user.
apps::SavedDevicesService tracks USB devices that have been selected by
the user in the context of a given extension. Devices that can be
identified accurately after they have been reconnected because they have
a serial number are written out to ExtensionPrefs. All others are only
remembered until they are disconnected.
A new OnDisconnect observer function has been added to UsbDevice to
enable this monitoring.
BUG=346953
Review URL: https://codereview.chromium.org/
580963002
Cr-Commit-Position: refs/heads/master@{#295708}