bp2build: fix running from clean checkout.

bp2build is a Soong mode that returns as soon as the BUILD files are generated. This causes it not generate the build.ninja file, which the Ninja process executing soong_build itself expects to produce as an output. If there isn't an existing build.ninja file generated from a previous build, GENERATE_BAZEL_FILES=true m nothing will fail.

This CL generates the expected files as a workaround, and also makes GENERATE_BAZEL_FILES=true skip the Kati/Ninja steps in soong_ui since they aren't needed.

Test: rm -rf out/ && GENERATE_BAZEL_FILES=true m nothing && m libc

Fixes: 178683777

Change-Id: I2515ef7961682d2be5f096ed24831cc185165a67