Ensure BrightnessTracker is only "start()"ed once

Here, we use mStarted to ensure that we dont reregister any broadcast
recievers. This was happening because initialize() would be called for
each DPC (of which there is one per display). This would cause start()
and stop() each to be called multiple times - causing a crash. There is
only one BrightnessTracker per device, however it needs to be
initialized within DPC because we need an initial brightness value,
which the first-initialised DPC provides us with.

Test: atest BrightnessTrackerTest
Bug: 253267570
Change-Id: I8f2aa74f5938cb370e97bf55dd97823be2a32627