Limit behavior to default DisplayPowerController.

Some code that DPC does is global in nature and should be
limited to the DPC built for the default display.

Test: atest frameworks/base/services/tests/servicestests/src/com/android/server/display
Test: Manually verify Android Auto doesn't have brightness contention
Bug: 178411081
Change-Id: I4d3293a9632ce19d4e9365ea3a7e6a0fe329133a