sensors: Allow target to override max range of proximity sensor

If a proximity sensor rarely or never reaches its maxRange, the
device will frequently lock up after the sensor is triggered (by
a call, for example), since it'll get stuck in the "active" state.

Set TARGET_PROXIMITY_SENSOR_LIMIT := <value> with the distance
from which the sensor will turn off to override those cases

Change-Id: I31fa2e5b9bd40ed6d80a1d5b0c147ed265f94c3a