wifihal: Enhance wifi_add_or_remove_virtual_intf to set interface mode

Considering wifi_add_or_remove_virtual_intf invoked with create=true:
if (interface already exists),
    try setting interface mode and returns.
else
    try creating interface with specific mode and returns.

This combines two functions into one API to minimize impact to
libhardware_legacy wifi_hal.h.

CRs-Fixed: 2639067
Change-Id: I2aac929adec64038c39a3163673d3532cf0daf12