prune out empty bazel depsets

Instead of using a sentinel file that was created under bazel_workspace
prune them out. The sentinel file was being created under bazel_workspace,
which would be deleted and thus rendered useless from the perspective of
not retriggering a build.

Test: `touch Android.bp` then `NINJA_ARGS="-d explain" m --bazel-mode-staging adbd_test`
Bug: 265155778
Change-Id: If5d8f0ea7f3b8828fda0646faafd1a621f0cb27c