Avoid rewriting soong_build outputs if unchanged

This changes bp2build codegen, symlink forest generation, and
soong_build so that they do not rewrite output files if the contents are
unchanged.

Bug: 266983462
Test: m droid
Test: canonical_perf.sh benchmarking
Test: Manually verified that rerunning analysis did not regenerate
out/soong/workspace/prebuilts/sdk/BUILD.bazel unless contents changed

Change-Id: I5ec227df7a32b53c7fa0d741fb1403a51931024b