frameworks/native: Add support for custom layers in Surfaceflinger

  - Check for the EXT_ONLY layer and add only that layer to external
    list and do not add that layer to the primary list
  - Check for INT_ONLY layer and remove that layer from external list
  - These are added to support EXTERNAL_ONLY & INTERNAL_ONLY feature

Change-Id: I8f84eeacebf97d44446f319f69b8a73e56f2cb6d