Alter the APS default block

[jrconlin]

from https://bugzilla.mozilla.org/show_bug.cgi?id=1413180#c4

This should be fixed on the autopush server. 

The default aps block should read:

{
    "mutable-content": 1,
    "alert": {
        "loc-key": "SentTab.NoTabArrivingNotification.body",
        "title-loc-key": "SentTab.NoTabArrivingNotification.title",
    },
}