Support vendor AT command and response for Hfp Client in bt stack

Add necessary interface in system/bt to support sending vendor AT
command. Also passing unknow events(vendor specific events) up for
further processing.

Bug: 132813146

Test: local test with kichensink to send vendor at command and receive
response. Also run unit test
atest BluetoothHeadsetClientStateMachineTest

Change-Id: Id460eaf00be269d15e6baf0fd30bdbf8e57df78d