Add Android backend

Add Android backend

The Android backend provides basic EGL/GLES2 graphics, where everything
is always composited. Overlays are not used. Input is stubbed, therefore
there is no input yet.

This adds the first C++ source file into Weston compositor. The Android
gralloc and fb HAL glue code to the Android EGL library is in C++, and
there is no way to access it from plain C. We have a simple wrapper to
the required C++ class API. Android forces the C++ file name extension
to .cpp.

The android backend is compiled by default. However, all Android
specific calls are protected with #ifdef ANDROID, so it will build also
without Android headers. The binary produced without the Android build
system is useless, but allows build-testing generic Weston changes.
Therefore the android backend is not installed.

Signed-off-by: Pekka Paalanen <ppaalanen@gmail.com>