egl_dri2: add support for Android

egl_dri2: add support for Android

Add platform_android.c that supports _EGL_PLAFORM_ANDROID.  It works
with drm_gralloc, where back buffers of windows are backed by GEM
objects.

In Android a native window has a queue of back buffers allocated by the
server, through drm_gralloc.  For each frame, EGL needs to

  dequeue the next back buffer
  render to the buffer
  enqueue the buffer

After enqueuing, the buffer is no longer valid to EGL.  A window has no
depth buffer or other aux buffers.  They need to be allocated locally by
EGL.

Reviewed-by: Benjamin Franzke <benjaminfranzke@googlemail.com>
Reviewed-by: Chad Versace <chad@chad-versace.us>
[olv: with assorted minor changes, mostly suggested during the review]