move Runtime.freeMemory and totalMemory to builtin.cpp

This allows them to be shared between the Avian and Android class
library builds.

This commit also disables the URL test in Misc.java on Android, since
it's known to fail, and we still want to know whether the other tests
pass.
Merge pull request #259 from dicej/totalMemory

move Runtime.freeMemory and totalMemory to builtin.cpp