I want to implement this feature in my apps , so that I can send messages to all my users and bring them into the app again.
My question is as notification ads are banned, will this feature breach any google policies , as this is kind of self-advertisement.
You are allowed to broadcast notifications to the user, so long as it’s directly related to a feature of the app.
So, for example, you could have a notification displaying “Your energy bar has re-filled!” where they are directed back to your game.
Here’s a quote from the new content policy regarding the subject:
Apps and their ads must not display advertisements through system level notifications on the user’s device, unless the notifications derive from an integral feature provided by the installed app. (e.g., an airline app that notifies users of special deals, or a game that notifies users of in-game promotions).
Thanks man 
This is something I’ve wanted to do, but never got around to - primarily because the implementation was too complicated - with Admob (doesn’t help that they keep “improving it” - so now Google Cloud Messing GCM is now Google Game Services etc.).
I also noted that there seems to be a limit to how many users you can notify - so for GCM it was 3000 or something … I might be mistaken.
And then there were alternatives - parse.com and urbanairship.com - which have pretty high limits (so if it is successful app you can probably pay them if you have such high number of users).
But I think there is some logistical thing there - i.e. which complicates it - for example with GCM it seems you had to SPECIFY who each of the users will be or something - maybe parse.com, urbanairship.com or the new Google variant of GCM has some simple way of “broadcast to all users” method - which I am assuming would keep track of users who have the app installed (but how would they keep track of if they have uninstalled the app if the device has no internet etc. and so on …).
Seems like really complicated stuff …
A variant on that maybe to do your own “notifications” - using Alarm or Broadcast Receiver or some such stuff - if anything maybe just issue one or two alarms may give good results (for user re-engagement).
The Google policy is not an issue - if you are awarding some type of currency for users coming back to the app etc. you could just issue a notification that “come back to the app today - there is a treasure trove of coins waiting for you” or some such thing - there you are giving a real feature to the user, and notifying the user would seem like a reasonable use of this thing …