Updater: Allow specifying a new device name

* To allow transitions from e.g. variant->unified build
* Example process, using klteusc->klte as the example:
 - Set ro.updater.next_device to klte in klteusc's tree
 - Wait for build to roll out with ro.updater.next_device
 - Pull klteusc from build roster

Change-Id: I3c70d54de3f9e036cd8700edc4ee0b11093740d2