Fixed a leak in GeofenceHardwareImpl.java.

Bug: 18542685.
This CL includes two changes:
* Fixed a leak of DeathRecipient when geofences are removed from the
  hardware.
* Avoid creating more DeathRecipient than needed. Use the underlying
  binder object instead of the callback object to tell if they are the
  same. So if the client passes the same callback instance to
  GeofenceHardwareImpl, only one DeathRecipient is created.

Change-Id: I7809e4bc04df4f9e3590de98a03178b276c821ea