Add manual activation area configuration option

Add manual activation area configuration option

Add a per-output "activation-area" configuration option that can be
used to define the activation area for applications that are not
using panels.

Notes:
- A new surface is not created for the given activation area, so
  apps that are not opaque will show the background.  After some
  thought, this seems like acceptable behavior, but it is possible
  that I am missing something.
- At present setting the activation area explicitly disables use of
  any panels, this may not need to be the case and some discussion
  with the AGL community with respect to requirements is likely
  needed.
- It is likely that this feature should be done via a agl-shell
  protocol call instead of via configuration, but doing so will
  require some thought as to the interaction with panels and how
  configuration errors would be communicated back to a client.

Bug-AGL: SPEC-4588

Signed-off-by: Scott Murray <scott.murray@konsulko.com>
Change-Id: I6cdae659f5f7636dc94121a69666b905abda3be3