Run the charger in the healthd domain.

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: I3fb6d3cd298dbec15165bf02496b2388a55f4b59
Signed-off-by: Stephen Smalley <sds@tycho.nsa.gov>