New call-in/-back functions for Controller to do vendor-specific shutdown (1/2)

A pair of asynchronous call-in and call-back API are added into the Host
Controller Vendor Lib interface (bt_vendor_lib.h). The caller calls this new
call-in function to inform the vendor module to perform vendor-specific
shutdown process (e.g. send HCI_RESET to BT Controller) before the caller calls
for interface cleanup() function. The vendor module is responsible for calling
call-back function to notify the caller completion of vendor-specific shutdown
process.
bug 7390787

Change-Id: I29e3c94ff78e17cdd7a06b44ae537e3b0ad1516d