do not set sdk_path in config.py if using system-provided aapt

do not set sdk_path in config.py if using system-provided aapt

By not setting sdk_path when /usr/bin/aapt is found, sdk_path then defaults
to $ANDROID_HOME when its used.  Since in this case, aapt will be used from
the system path, using aapt entirely ignores sdk_path.  If the user runs
`fdroid build` in this setup, sdk_path will be $ANDROID_HOME, so it should
check the env vars for it, but maybe that doesn't actually work like that
yet.