SensorService now always clamps the requested rate

Requested rate will be clamped to the minimum rate and then
to 1ms. Previously we would return an error if a lower
rate was asked. The SensorManager documentation wording
allows this change.

We do this to get more consistancy between all the sensor
drivers / HALs

Change-Id: I199f76486fb76ccbb11e7280460a03726c767e84