Add support for skipping just kati

The existing --skip-make flag disables both the config step and the kati
step in the build. Add support for a --skip-kati flag that skips just
the kati step, and refactor things so that the logic is shared between
these two.

Bug: 174315599
Test: TARGET_PRODUCT=aosp_arm64 soong_ui --make-mode --skip-kati;
      (verify soong.variables is regenerated)
Change-Id: I75b1910fc1c12fcda130e37b7bc4c050131c7b33