Always merge build files

Previous behavior:

- Packge not listed in bp2buildKeepExistingBuildFile:
    - Use bp2build generated build file
- Package listed in bp2buildKeepExistingBuildFile:
    - Use handcrafted build file even if there were allowlisted bp2build
      modules in the same package.
- Package listed in bp2buildKeepExistingBuildFile and a soong module has
  a bp2build: { label } attribute:
    - Merge the handcrafted and bp2build generated build files

New behavior:

- Packge not listed in bp2buildKeepExistingBuildFile:
    - Use bp2build generated build file
- Package listed in bp2buildKeepExistingBuildFile:
    - Merge with bp2build generated build file.

Bug: 234167862
Test: ./build/bazel/ci/bp2build.sh
Change-Id: Ifbaf4f8f0f5158b5b2bd6d534eb2311e2e5f399b