[fix] Add app extension compatibility

[bsmith11]

#290

This is the implementation that was used in PINCache: pinterest/PINCache#72

It has since been removed, but as far as I can tell, only because the feature requiring [UIApplication sharedApplication] was removed as well:
pinterest/PINCache#99

[demerino]

Thanks for putting this PR together - we'll take a closer look in the next few days.

Because these changes return false when isAppExtension - we'll also want to add something to the analytics metadata to mark these events as being from an app extensions so they don't pollute our data.

[skunkworks]

Thanks for opening this PR @bsmith11 . Your changes seem reasonable, but I think we have a little bit of work on our end to make analytics from app extensions more coherent. Our current plan is to merge this to a separate branch and work off of that before we merge to master.