Add statsd atom for reboot escrow success

In order to report the success or failure of the RebootEscrow HAL, add an atom
which can indicate when it succeeded or failed.

Bug: 63928581
Test: Trigger OTA flow and watch reboot with "statsd_testdrive 238"
Change-Id: I065e5660d9fd1f2324058a94cc3d624fdf125a89