[bt][mgr] Start bt-snoop when bt-mgr starts up.

 - If bt-snoop is enabled, it should be running anytime the Bluetooth
   stack is active and continue running in the event of a crash to
   the Bluetooth subsystem.
 - Autostart of bt-snoop can be configured in bt-mgr's config file.

BT-703 #done
TEST=1. Confirm that bt-snoop is not running with `cs`
     2. run bt-cli to bring up Bluetooth stack
     3. Confirm that bt-snoop is running with `cs`

Change-Id: I96d822fba8b2b67c253a091e8b7cfbc9873b1850
5 files changed