Only monitor recognized uevent devices.

system_server can potentially monitor uevent devices that are
of no use to it. For instance, an accelerometer implementation
as uevents. This would cause the process to be busy when
unnecessary. If a device cannot be classified, don't monitor it.

Change-Id: Ib2c93105e7d746d9c1a7414bea8bab3fb8c0b70a