[FIX] Push notifications with lower priority for Android devices

[ceefour]

According to https://www.npmjs.com/package/node-gcm#usage and recommendation in https://developer.android.com/training/monitoring-device-state/doze-standby

Continuation of #15803 (comment)

Proposed changes

According to node-gcm, it should be like this:

var message = new gcm.Message({
    collapseKey: 'demo',
    priority: 'high',
    ...

Compared to what gcm.js does as of v3.6.3:

const message = new gcm.Message({
		collapseKey: notification.from,
		//    delayWhileIdle: true,
		//    timeToLive: 4,
		//    restricted_package_name: 'dk.gi2.app'
		data,
	});

Issue(s)

Fixes #15803 (comment)

Improves upon #17357, #17907, and RocketChat/Rocket.Chat.ReactNative#2507.

How to test or reproduce

From @xtilyn:

Right now when phone enters doze mode, android app is not able to receive notifs.

You can test this by forcing doze mode using adb
adb shell dumpsys deviceidle force-idle

Then, if you send a message, the android client does not receive it until you disable doze mode with
adb shell dumpsys deviceidle unforce

Screenshots

Types of changes

• Bugfix (non-breaking change which fixes an issue)

Checklist

• I have read the CONTRIBUTING doc
• I have signed the CLA

Changelog

fix(push): Set push notification priority to 'high' for FCM

Further comments

cc @sampaiodiego