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
diff --git a/include/gui/Sensor.h b/include/gui/Sensor.h
index e696d63..2de07b1 100644
--- a/include/gui/Sensor.h
+++ b/include/gui/Sensor.h
@@ -63,6 +63,7 @@
     float getMaxValue() const;
     float getResolution() const;
     float getPowerUsage() const;
+    int32_t getMinDelay() const;
 
     // Flattenable interface
     virtual size_t getFlattenedSize() const;
@@ -81,6 +82,7 @@
     float   mMaxValue;
     float   mResolution;
     float   mPower;
+    int32_t mMinDelay;
 };
 
 // ----------------------------------------------------------------------------