android: add CleanSpec.mk

The file contains rules that are executed on incremental builds. This
way one can avoid doing a full clean and ensure that the new object
(library) is correctly build. Let's also cleanup the headers.

Inspired by the work of Chih-Wei Huang, from the Android-x86 project.

Signed-off-by: Emil Velikov <emil.l.velikov@gmail.com>