Add a soong-only mode to soong-ui

The previous --skip-kati flag could be interpreted as "do not run kati
to re-generate ninja file". Add a more specific flag for the "soong
only" build use-case, where we do not load the kati-generated ninja
files at all.

Bug: 189187214
Test: build/soong/soong_ui.bash \
  --make-mode \
  --soong-only
  --skip-soong-tests \
  TARGET_PRODUCT=mainline_sdk \
  SOONG_ALLOW_MISSING_DEPENDENCIES=true \
  SOONG_SDK_SNAPSHOT_VERSION=unversioned \
  SOONG_SDK_SNAPSHOT_USE_SRCJAR=true \
  out/soong/mainline-sdks/art-module-sdk.zip
Change-Id: I91abbd28af517d4b550ebc6d88fd64947caf9545