Metadata leak in push notifications

[chadyj]

Problem

Reported by @hesterbruikman:

If a message sender has their mobile OS language settings set to a supported language (e.g. Korean or Spanish) the notification of the recipient is shown in this language, regardless of the anguage the recipient has set in their OS. E.g. Jinho sends a message to Philio while Jinho's phone is set to Korean, Philip receives a notification in Korean while his phone is set to English.

Acceptance Criteria

Push notifications should not reveal metadata

cc @mandrigin @corpetty @rasom @pombeirp @corpetty

[pedropombeiro]

This is yet another instance where we see it's better to rely on the data field for the payload instead of notification field (another being e.g. #3451). We should be sending a silent notification (only data filled in) and getting that to trigger an intent on our mobile app, which would be responsible for decoding the payload and actually displaying a localized message.

[chadyj]

@pombeirp Thanks for taking a look. Will you be able to take this?
Looks like there is at least a quick fix here and perhaps a broader PN improvement?

[pedropombeiro]

I'll take a look and try to make it backward-compatible with older clients.