health 2.0: Check health_loop status for passthrough usage

health HAL is used as passthrough in recovery mode and hence
healthd_main() is not invoked. Add checks in
healthd_battery_update_internal() and healthd_register_event()
to return when health_loop is NULL.

Test: fastboot getvar battery-voltage works in fastbootd.
Bug: 144299329

Change-Id: I0a975a4bfe942813f9c5b3936df91cc3d8fa9674