Added nullability annotations on return types and method params.

[autonomousapps]

trafficstars

I feel this is a good first step towards dealing with other possible issues.

[autonomousapps]

@serggl do you know why the travis builds have failed?

[serggl]

Travis failure:

* What went wrong:
Execution failed for task ':library:checkstyle'.
> Checkstyle rule violations were found.

[raazronik]

@ramzik