Android-components: Add support for checking if a push subscription exists

[jonalmeida]

For web consumers, the API for WebPushDelegate.onGetSubscription is expected to function differently from how we implemented it.

Querying that API, should check if a subscription exists and return it, otherwise return null.

Instead, we were eagerly creating a subscription when that query was made.

This resulted in sites being given a push subscription without user permission.

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We don't have a straight-forward mechanism to query if a subscription already exists, but we can check if the scope and endpoint exist. That should be enough to satisfy this API without creating a new subscription during the query and _only then_ invoking the subscribe API to get-or-create.

This might also be the cause of https://github.com/mozilla-mobile/android-components/issues/7161 but let's fix this first and verify that after.

[jonalmeida]

cc: @TitanNano for reference

[jonalmeida]

@TitanNano the next Fenix build should have the fix for this. If you get a chance to test it out, please let me know. :)

Thanks for filling and triaging all these WebPush bugs!

[TitanNano]

@jonalmeida I can confirm in Fenix Nightly 200530 06:01 (Build #21510609) with AC 44.0.20200529205416 PushManager.getSubscription() now correctly returns null if no subscription exists and does no longer create unauthorized subscriptions.