fix(MainActivity): Check if SensorManager has been already collected on unregister

Otherwise sometimes on OnPause a NullPointerException will be thrown

Closes https://github.com/psycholold/SensorReader/issues/20
Merge pull request #22 from psycholold/20-null-pointer-on-stop

fix(MainActivity): Check if SensorManager has been already collected on unregister
fix(SensorReader): Correct timestamp in log and no output buffering

Fixes the timestamp in the output. Also removes buffering in the SensorReader for faster
updates.

Closes https://github.com/psycholold/SensorReader/issues/13 and https://github.com/psycholold/SensorReader/issues/16