Avoid null pointer when in getAmbientBrightnessStats

Check whether the tracker has been started before
trying to return stats.
Add unit test to check empty lists are returned
rather than null.

Bug: 76008231
Test: atest BrightnessTrackerTest
Change-Id: Icc7ef0ee43a95eb182cb1b0b9452f122363e67ba
2 files changed