hwc2: Add support for multiple displays

- Add support for an additional display which can be either a builtin
  display or pluggable display.
- Restructure HWCDisplayPrimary as HWCBuiltIn and HWCDisplayExternal
  as HWCDisplayPluggable.
- Refactor hotplug handling. Iterate through all pluggable displays,
  and check present connection status. Connect/disconnect a given
  display if connection status has changed.

CRs-Fixed: 2200594
Change-Id: Ibd2f93ddfbe72af58220691ad481469c8fdcbf86
17 files changed