feat(fcm): Support `proxy` field in FCM `AndroidNotification` (fireba…

…se#1084)

* add 'proxy' to AndroidNotification

* fix: removed PROXY_UNSPECIFIED from AndroidNotification.Proxy

* Update src/main/java/com/google/firebase/messaging/AndroidNotification.java

fix comment AndroidNotification.setProxy

Co-authored-by: Jonathan Edey <[email protected]>

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Co-authored-by: Jonathan Edey <[email protected]>