I don't understand why Android includes this lib in their source but makes it internal so you have to provide your own copy if you want to use it in your app: https://android.googlesource.com/platform/external/libphonen...
Sad that nowadays a statement like that isn't even considered ironic :). A cross-platform library handling an ugly necessity of development has to fight for its life in a sea of "growth hacking 101" links and the like.
If Google is the only consumer, then Google only has to test internally that they didn't break something with an update. If tons of apps are consumers then they need to either test with many more apps, or worse just simulate buggy old behaviour for apps to keep things consistent (see Windows/Win32/etc).
If apps house their own copy of the library, then that version is static for that app's context, which likely means it won't break randomly in the future. That's good for the app developer (better back and forward compatibility), better for Google (less testing, less maintaining compatibility), and less headaches for consumers (as apps won't break as often when updating Android).
This reminds me of one of my favourite quotes: "If something in software doesn't make sense, then assume it is there for compatibility reasons." Which I think is a Raymond Chen quote that I've mangled.
We were trying to use Twilio and Authy on an application but they handle phone numbers in a different way. Say you accept a phone number from Argentina, to call mobile phones from outside the country you need to add a 9. Twilio accepts that 9 and correctly sends SMS or calls to that number but Authy only accepts the local number format without the 9 and have a big IF to add the 9 internally. So, if for some reason the application must use a phone number in Twilio AND Authy you will need to accept two phone numbers from the user (that are the same in different formats), which is... annoying.