Convert mesa host builds to Android.bp

As this is the first conversion in this project, it sets up a new soong
namespace (to be equivalent to the logic around
BOARD_USE_CUSTOMIZED_MESA / BOARD_GPU_DRIVERS).

It also sets up a cc_defaults equivalent to Android.common.mk, and an
aosp-gen-bp.sh script that autogenerates some Android.bp files to be
used when mesa is updated.

Bug: 122332854
Bug: 130696912
Bug: 130723227
Test: lunch aosp_cf_x86_phone-eng; mmm external/mesa3d
Test: treehugger
Change-Id: Ibe5a751e20be1b37be612c9940b49e3c8a88a900
diff --git a/Android.common.mk b/Android.common.mk
index a8a6911..cc02e6e 100644
--- a/Android.common.mk
+++ b/Android.common.mk
@@ -21,6 +21,9 @@
 # FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER
 # DEALINGS IN THE SOFTWARE.
 
+# ! Whenever updating this file, make sure to keep mesa_common_defaults in the
+# ! Android.bp up to date.
+
 ifeq ($(LOCAL_IS_HOST_MODULE),true)
 LOCAL_CFLAGS += -D_GNU_SOURCE
 endif