Make multiproduct_kati call soong_ui.bash .

This serves to not link parts of soong_ui (and eventually soong_build)
into a separate, weird binary. This is in turn good because they contain
any number of global variables and no one really thought about what
happens when two instances are executing at the same time in the same
address space.

This comes with a slight performance hit: 5 aosp_* projects build 152
seconds instead of 146. I suppose this is a price worth paying for a
clean design?

Test: presubmits.
Change-Id: I5623dcab2290f0fc392dd2ede597b9794a3d2a4e