Clear caller id when reading device config

Reading the DeviceConfig with the gmscore call id will result in a
security exception.

Also clear the caller identity when before calling into locksettings
to armRebootEscrow. This is inline with the other usage for RoR
preparation.

Bug: 183475757
Test: trigger RoR from gmscore
Change-Id: Ifc65625fc7aba05d016c127cf6672922a5ffb000