Add HCI user channel based bluedroid vendor adaptation

This libbt vendor library creates and binds an HCI user channel
instead of talking to a serial port file descriptor. HCI frames
generated by the Android Bluetooth HAL are passed down as is to the
Linux kernel Bluetooth drivers through those sockets.
By pushing all the hardware dependencies down to the kernel, this
architecture supports all existing Linux Bluetooth drivers with
one single AOSP Bluetooth HAL implementation, Bluedroid.

Change-Id: I7197016b556e5d985843dd4f626db5d0f596f996