adb: add libusb implementation for Linux/Darwin.

Add a libusb-based implementation alongside the existing native
implementations, controlled by the ADB_LIBUSB environment variable.

Windows will need more work for the usb driver.

Bug: http://b/31321337
Test: python test_device.py on linux/darwin, with ADB_LIBUSB=0 and 1
Change-Id: Ib68fb2c6c05475eae3ff4cc19f55802a6f489bb7