Only load default lightsensor for Brightness Tracker

Previously, we were setting new light sensors each time the display
changed. This meant that during transitions between displays, multiple
light sensors could get loaded since we create temporary displays during
transitions. This cl ensures we are only using the light sensor of the
default display, since, for now, BrightnessTracker can only take one
lightsensor.

We also only load the brightness tracker for the default dpc, since
other displays will not use this.

Bug: 248261521
Test: com.android.server.display
Test: dumpsys display | grep -A40 "active connections" and ensure there
is only one instance of a brightness tracker connection.

Change-Id: If0397af675caed0568bed06d82f7d925002c8df8
Merged-In: If0397af675caed0568bed06d82f7d925002c8df8