This change allows the InputManager to keep track of what input
devices are registered with the system and when they change.
It needs to do this so that it can properly clear its cache of
input device properties (especially the key map!) when changes
occur.
Added new API so that applications can register listeners for
input device changes.
Fixed a minor bug in EventHub where it didn't handle EPOLLHUP
properly so it would spam the log about unsupposed epoll events
until inotify noticed that the device was gone and removed it.
Change-Id: I937d8c601f7185d4299038bce6a2934fe4fdd2b3
af9e8d3818