Clean up sdk building, use built xmllint

Instead of relying on xmllint from the path, with could come from the
host, or sometimes a built version, always use the built version
explicitly.

This does some general cleanup of these make rules as well, using fewer
global variables inside make rules, and cleaning variables up at the end
of the file. I added some missing dependencies and removed some uses of
$(TOPDIR) too.

I did not clean up the uses of `zip` that make these rules
non-deterministic, but I filed b/117562080 for that, since it could use
some larger refactoring.

Test: lunch sdk_arm64-userdebug; m sdk sdk_repo dist
Change-Id: Ia0736d5fc7475603cf31f385b3ada11d9e775259