Prevent EventHub from adding input device twice

When Android first starts up, it scans /dev/input for input devices.
In some rare instances, the EventHub gets another notification that
some device nodes are created. It then proceeds to add the same
input device again. This causes the system to get two events per
touch or key stroke.

This adds a check to prevent adding the same device if the operation
is triggerd by inotify.

Change-Id: I68b02594f1c7f14067611735db0b3763378ec7ea
Signed-off-by: Daniel Leung <daniel.leung@linux.intel.com>