Rationale: on Android one must use UsbManager, to access any
USB device. As a result, libraries like libusb can only use file descriptors
that are provided by UsbManager.
libusb has an API to use such file descriptors: hid_libusb_wrap_sys_device.
Having hid_libusb_wrap_sys_device currently is the only way to make hidapi
work on Android without root access and without custom Android builds.
Relevant info: https://github.com/libusb/libusb/pull/830/files
Historically, there was no libhidapi.so for libusb backend,
and no good reason not to be consistent with the existing build system.
Also, this will make it easier to enable libusb backend
for platforms like Windows/macOS, where native libhidapi.so already exist.
