hwc: Add secure display attribute

- External displays can be secure or non-secure based on HDCP
- Propogate this to the SF with the getDisplayConfig
- Non-virtual displays(pri and hdmi) are considered to be secure
- For WFD using V4L2 archiecture, read the HDCP status from the
  sysfs and update accordingly

Change-Id: I4aaf303f0770201b8ca4f8a1c578bed61054bcfb