Enable linux netlink event monitoring for Android OS platform services
Introducing the ANDROID_OS macro that is supposed to be set by clients
of libusb that are running on Android at the OS level.
If Android_OS is set (and HAVE_LIBUDEV is not) then linux netlink event
monitoring (and consequently hotplug functionality) is enabled. This
works for Android services or simple binaries running at the OS level as
they have the necessary permissions, in contrast to usual Android apps
(see commit
2f3bc98).
Note: the __ANDROID__ macro is set by GCC for all targets running on
Android (both OS-level services and apps).
Closes #1577
Signed-off-by: Nathan Hjelm <hjelmn@google.com>