Change request: using non null types for system services

[jakubvimn]

From the getSystemService method documentation:
"@return The service or null if the name does not exist"

So, if a name constant is used (and not a random string), the API guarantees to return a non-null object. So I think that the returned objects should have non-null types.

[pawegio]

It can return null if service is not available on the device from incompatible hardware or API level reasons, e.g. JOB_SCHEDULER_SERVICE is accessible since API level 21.

[jakubvimn]

That's true. But what about the services available from API 1?

[pawegio]

See #3