Add a unique input device descriptor.

The purpose of the input device descriptor is to make it possible
to associate persistent settings for each input device, such as the
keyboard layout.

The descriptor is a hash of the information we have about the
device, such as its vendor id, product id, unique id, name,
or location.

Bug: 6110399
Change-Id: Idb80f946819b3f0dbf4e661bb0a753dbc2b60981
10 files changed