Build partition images and SBOM seperately in case the build of SBOM...

Build partition images and SBOM seperately in case the build of SBOM introduces extra dependencies that included in partition images.

Bug: 272358980
Test: build/soong/tests/sbom_test.sh
Test: CIs
Change-Id: I99bd2357344f9a3d5827cf052b69e3e8545241c0