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>