Define equals and hashCode for InputDeviceIdentifier.

This CL fixes that physical keyboard layout is not changed when user
changes it in settings.  This happened because we compare
InputDeviceIdentifier instances by using Object#equals to choose
specified input device. However, one of them has been serialized and
deserialized, so it never be true.

Bug: 27747115
Change-Id: Ied84c510ccb8e2de919ba8bb326e0355a065e604