Resolve "Import pull request #1069 from microG (GitHub), FCM registration"
Description
Change USER_AGENT version from 1.3 to 1.5, store extra headers we get from Google, and pass along the needed headers for correct FCM registration.
Type of change
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Bug fix (non-breaking change which fixes an issue)  
How has this been tested?
Imported the diff, built microG with the provided pull request, installed new microG apk on S7 herolte latest /e/ dev, installed Signal, verified that Settings->MicroG->Google Cloud Messaging lists Signal, send a message to the /e/ S7, S7 showed a notification.
Checklist:
- 
My code follows the style guidelines of this project  - 
I have performed a self-review of the pull request  - 
I have commented my code, particularly in hard-to-understand areas  - 
I have made corresponding changes to the documentation  - 
New and existing unit tests pass locally with my changes  - 
Any dependent changes have been merged and published in downstream modules  
Closes #41
Edited  by Alexandru Chircu