Add null Activity check in PlayServicesLocationManager.getCurrentLocationData

[samzmann]

Overview

This PR addresses issue #217 in which app is crashing probably because activity returned by mReactContext.getCurrentActivity() is null.

As suggested in my comment #217 (comment) I added a null check in PlayServicesLocationManager.getCurrentLocationData.

If currentActivity is returned as null, we invoke the error callback with a new PositionError (ACTIVITY_NULL), and then just return to end the method execution.

Questions

1. I added the new ACTIVITY_NULL error to the PositionError class. But this is not a position error, more some underlying Android/React Native error (I don't know 😅). Adding this to PositionError is quick and easy, but to be precise and clean it would make sense to put it in another class, eg. UnexpectedError or something like that..
   Also this breaks the so called "feature parity with iOS", and makes the GeolocationError TypeScript type inaccurate.
   Any thoughts on that?

2. At first I tried to call the error callback with emitError, but the error was not reaching the JS. So I changed to error.invoke, and then the error was reaching the JS. I see that you made changes to how the error callback is called in the last two commit (741a353, 362bf80), and I am wondering if these changes are maybe a mistake?

3. Unrelated: I tried to run the e2e tests, but noticed the script does not exist in package.json. Maybe CONTRIBUTING.md should be updated?

Test Plan

I added the changes in my app with a patch and then hardcoded

- Activity currentActivity = mReactContext.getCurrentActivity();
+ Activity currentActivity = null;

to make sure that the null check was working and that the error was reaching the JS, which it did.

Then, I changed it back to what it's supposed to be

+ Activity currentActivity = mReactContext.getCurrentActivity();
- Activity currentActivity = null;

and made sure the location data was reaching the JS, which it did.

[michalchudziak]

Hey, thank you for the contribution! The code looks nice. I'll test it out and include it in the upcoming release.