find cmds from SDK build-tools in a more flexible way, on the fly

find cmds from SDK build-tools in a more flexible way, on the fly

This is a more flexible approach than testing for the complete SDK and
build-tools up front.  This will only test for the commands that are
actually being run, so that if you only have `aapt` installed, you can do
`fdroid update` without errors, but other commands will still give
appropriate errors.

This also makes the build_tools item in config.py optional, it is only
needed if you want to force a specific version of the build-tools.