base: Do not overwrite VIRTUAL_DISPLAY_FLAG_* values

* VIRTUAL_DISPLAY_FLAG_SUPPORTS_PROTECTED_BUFFERS is
getting read as VIRTUAL_DISPLAY_FLAG_OWN_CONTENT_ONLY.
This impacts the content seen on the VD device.

* Ensure that VIRTUAL_DISPLAY_FLAG_SUPPORTS_PROTECTED_BUFFERS
and VIRTUAL_DISPLAY_FLAG_OWN_CONTENT_ONLY are different
values to prevent this.

CRs-Fixed: 653794

Change-Id: Ic0ea99a554ab26f2ff941304d1c6809f976076ef