FCM/ADM router notification size limit check is inaccurate

[AzureMarker]

The FCM router (and any router which uses the common build_message code) only checks the notification size limit against the data that is being passed through, not the final payload:

autopush-rs/autoendpoint/src/routers/common.rs

Line 17 in 9cdb14a

if data.len() > max_data {

This check could allow payloads which have a small enough data size but too much overall data in the final payload. The check should be moved so it is done on the final payload.

[AzureMarker]

From testing, FCM only considers the data field when calculating the payload size.