Support default hbm metadata for non-internal displays

 We currently have HBMController for all the displays irrespective of their type,
 and each HBMController implicitly assumes that it has an associated metadata.

Bug: 265777465
Bug: 265186363
Test: Added a new unit test.
      atest com.android.server.display.DisplayManagerServiceTest#testHighBrightnessModeMetadataNonNull
      Manual testing with multiple device with virtual displays
      to make sure SystemUI works as intended.

Change-Id: I492265b1385f6121196f18e8a4152f4e2fc6ef54
Merged-In: Idf45b87782a56bcbc09fc37c69fcc13531edf76f