Added SensorManager.getMinDelay()

Exposed the new "min delay" sensor property through native and
java sensor apis. This allows the caller to know what is the
maximum rate at which a sensor can return events, or, if a sensor
works in "update" mode (events returned only when the value changes).

Also augmented SensorManager.regusterSensorEvent() so that it can
accept a value in microsecond in addition to the 4 constants already
defined.

Change-Id: If425e9979892666df8c989d7de3c362230fa19e0