Generate build_number.txt only once

build_number.txt was generated in runKati, and runKati is run multiple
times throughout the build. If a build number is not set, then one will
be generated using the current timestamp, meaning that multiple
different build numbers will be used in different phases of the build.

Instead, generate build_number.txt during SetupOutDir(), so that we
only have 1 build number.

Bug: 297269187
Test: edit a bzl file to force bazel to rerun, change the code so that use_fixed_timestamp=true during partition builds, m installclean, m bazel_sandwich, and observe that the bazel/make partitions are byte-for-byte identical
Change-Id: I5bf3bb6c78e7529749ca9275f67db3f2f9e66af2