sf: vds: Add support for secure virtual displays

1. Propagate secure flag on display creation

In SurfaceFlinger we have information that tells us whether a
display is secure or not. We need to propagate this information
when creating the corresponding virtual display, allowing us to
configure output buffers with the correct (secure) usage flags.

2. Use secure heap only for sessions that need HW level protection

Use MM heap only if the secure session needs hardware level
protection. At the present moment we are assuming that only displays
with the GRALLOC_USAGE_HW_ENCODER need hardware level protection.

Change-Id: I7e0d42ba3a81d1f5c42b1074e3018826b38b7a8d