Run the charger in the healthd domain.

The current policy assumes that the charger will run in the healthd domain.
Add a seclabel entry for the charger service to ensure that it runs in the
healthd domain even when the charger is a separate binary.

Change-Id: Ib79e60425fa2244a5716327f45e63d2c216ad2cc
Signed-off-by: Stephen Smalley <sds@tycho.nsa.gov>