bp2build: allowlist //external/brotli/... and build libbrotli. am: 60da00eb...

bp2build: allowlist //external/brotli/... and build libbrotli. am: 60da00eb am: 7490e0c1 am: 3d09d173 am: 0f5c8190

Original change: https://android-review.googlesource.com/c/platform/build/soong/+/1814446

Change-Id: I715be4fbbf89bdc60699a4fc80cc26eff89db6d1