Prevent creating AmbientLightModeMonitor with no sensor.

Provider<LightSensor> will be created even in the case LightSensor is
null. This changelist prevents issues stemming from this by checking the
fetched value before using it to create an AmbientLightModeComponent.

Test: manual - ensured build runs without sensor
Bug: 432733048
Flag: EXEMPT bugfix
Change-Id: I583bef8517167f11b720886ac34e7a9c6538e070