Reboot device when rolling back staged installs

Staged installs require a device reboot to be fully activated, same
with the rollbacks. We wait for the session_updated broadcast
indicating the rollback session is written and then we reboot the
device to apply the staged rollback.

Test: TBD
Bug: 120598832
Change-Id: I139b352cc8cce2016d3c9d74e37b1060e2bd4511