Fix OUT_DIR lookup in mixed builds.

In mixed builds, Bazel operates in <root>/out/soong/workspace, but the real
OUT_DIR is just <root>out/, not <root>/out/soong/workspace/out. This CL
ensures that the correct OUT_DIR is passed to mixed builds's bazel by
making OUT_DIR absolute.

Also add STANDALONE_BAZEL to build_test's Bazel smoke test.

Test: TH
Change-Id: If8b4058d43bf821f5196016f9b0ad38edfe18716