Add ro.slim.device for OTA porpuses

  * Due to unified builds disabling some id props, Slim OTA wasn't working
     working properly. Lets add a new prop just for our needs

Change-Id: Ic2b3acb3dd4944160449f228cb69ec2c9f08775c
Signed-off-by: Josue Rivera <prbassplayer@gmail.com>
Add ro.slim.model for OTA purposes

Change-Id: I5d4baf0548e098d629e900580758a6d582d3cc2b
Update roomservice.py