Do not create a build.Config in multiproduct_kati.

This mostly cuts the dependency from multiproduct_kati to the build
package; the remaining logic is SetupSignals which will be dealt with in
a followup change.

This requires some code duplication (e.g. ensureFileExists() and
detectTotalRAM()).

This removes a Finder invocation added in aosp/452598; it looks like
it's not a measurable performance hit. The quoted half a second does not
matter much in build_test{1,2,3,4} which take an amount of time measured
in hours.

It also makes ANDROID_QUIET_BUILD unsupported for the UI of
multiproduct_kati; the function of that variable seems to be to suppress
the output from build actions, which don't exist in multiproduct_kati so
it should not matter. In addition, I checked whether anyone sets this
flag for multiproduct_kati and it doesn't appear to be the case.

Test: Presubmits.
Change-Id: I1c2901e1ea69c3a35429e6c967a7bba845676ef7