Make LogicalLight @Nullable: part one (external)

This helps to simplify the internal logic of LightsService to not have
to worry about sending bad values down to the HAL if system services use
lights that don't actually exist.

Bug: 151113302
Test: atest google/perf/boottime/boottime-test
Change-Id: I0512c71e376c64095270b8e70e9754fe0f81210e