usb: Expose persistent vendor property for USB-C Dp Alt Mode

The change exposes persist.vendor.usb.displayport.enabled
to enable/disable USB-C Dp Alt Mode i.e. DisplayPort over USB-C.

By default persist.vendor.usb.displayport.enabled is to 0 in
user builds and 1 in all other build variants.

Bug: 277107943
Change-Id: I596d3af52f2a813f5f78c4f9b77b6ae3dd8a161b
Signed-off-by: Badhri Jagan Sridharan <badhri@google.com>