Bug Fix: Android app not focused when using urlHandler or customActionHandler

[jehartzog]

Fixes #76.

I went tracing through this and iterable/iterable-android-sdk to figure out why we had some many issues with our Android app not bring brought into focus. I'll lay out what I found here, starting at the top level.

1. The actual config.customActionHandler you assign is fired here, via RNEventEmitter. Note that nothing is actually done with the return value from your JS handler.

2. The event which executes the above is triggered in native Android here. The key issue here is this function always returns true, which tells the native code that any custom urls/actions are always handled. This also happens for urls.

3. The above function is called here, where if we have a customActionHandler defined, then we return the result of the above (which as we saw, is always true). Otherwise we return false.

4. The problem comes here, where we execute the action, and if handled === true than we don't bring the app into focus. I believe this may have been doing sensibly for native Android apps, where they want to intercept a URL/action and can easily bring the app into foreground. However this is not the same for RN, as this code need to be run on native side.

I initially created a fork of iterable/iterable-android-sdk which fixed the issue, but I later realize it would likely to be easier to fix for the RN use case, without breaking changes to the existing iterable-android-sdk, if we made the changes in this PR.

[vbabenkoru]

Thank you for the PR! Top notch investigative work @jehartzog! You're absolutely right: with RN, we can't assume the app calls startActivity, because RN apps are generally single-activity apps, and it's ok to send a launcher activity intent.
Thank you for the contribution. Very helpful.

[roninopf]

Thank you @jehartzog! I've gone ahead and merged this, and it will be released soon.