BlackBerry: Update threshold for close detection

The current algorithm for close detection in QProximitySensor will not
work for sensors whose output is not binary (ie, close/far). The new
algorithm will use the threshold of 8cm when the output is not binary.

Change-Id: I2173e9afae0a2d29434e092e1afc09d1c6f5647c
Reviewed-by: Bernd Weimer <bweimer@blackberry.com>
Reviewed-by: Fabian Bumberger <fbumberger@rim.com>