contrib: add sample Android.bp to build within an Android tree

contrib: add sample Android.bp to build within an Android tree

Add an Android.bp file for Soong, the Android build system, to build
the library including C++ bindings along with all the CLI tools.

This reference Android build file will live in a contrib/ folder to
indiciate it is a less-maintained part of libgpiod. It will need to
be moved to the root directory in order to use it, though, as Soong
doesn't let Blueprint files reference sources in a parent directory.

  error: external/libgpiod/contrib/Android.bp:5:1: module "libgpiod" variant "...": Path is outside directory: ../include
  error: external/libgpiod/contrib/Android.bp:5:1: module "libgpiod" variant "...": Path is outside directory: ../bindings/cxx
  error: external/libgpiod/contrib/Android.bp:5:1: module "libgpiod" variant "...": Path is outside directory: ../lib/*.c
  error: external/libgpiod/contrib/Android.bp:5:1: module "libgpiod" variant "...": Path is outside directory: ../bindings/cxx/*.cpp

Signed-off-by: Benjamin Li <benl@squareup.com>
[Bartosz: add the build file to the release tarball generated by 'make dist']
Signed-off-by: Bartosz Golaszewski <bartosz.golaszewski@linaro.org>