Handle light sensor NullPointerException.

Light sensor is not available on all devices so add a check for
availability before starting the ambient light mode monitor.

Test: atest AmbientLightModeMonitorTest#shouldNotRegisterForSensorUpdatesIfSensorNotAvailable
Bug: 203691187
Fix: 203691187
Change-Id: I1f77f1337f24409986029bf018b6e6f43857db87
2 files changed