base

clear caller identify before sending system update notification

Because DeviceAdminReceiver is protected by BIND_DEVICE_ADMIN permission,
in order to send broadcast to it, we need to clear the caller's identity
and call sendBroadcastAsUser() as system.

Bug: 20213644
Change-Id: Icc7b239b9005e286012ade6580ec92a0a57198e0