Create separate Android.mk for main build targets

The runtime, compiler, dex2oat, and oatdump now are in seperate trees
to prevent dependency creep.  They can now be individually built
without rebuilding the rest of the art projects. dalvikvm and jdwpspy
were already this way. Builds in the art directory should behave as
before, building everything including tests.

Change-Id: Ic6b1151e5ed0f823c3dd301afd2b13eb2d8feb81