Allowlist build files in external/python/absl-py

Needed because we import absl-py in our bazel.workspace file.
It only worked before in b, not mixed builds, because b was
run from the real android tree instead of out/soong/workspace.

Bug: 234449134
Bug: 249685973
Test: Presubmits
Change-Id: Ie6985582c55a4a95338fb3a8ff7b13ddb522c391