Implement ISensorsCallback

Implement the ISensorsCallback interface in the SensorDevice. This
interface is responsible for handling callbacks from the Sensors HAL,
such as when a dynamic sensor is connected or disconnected.

Bug: 111070257
Test: Compile, configured system to use sensors@2.0 and ensured that
      a_sns_test received events

Change-Id: I180e05db29e0e1d008fb661e6cf6e3b552a1b0b6