The soong primary builder action lists all the Android.bp files
as dependencies via a depfile. The top-level one doesn't need to be
listed as an input explicitly. I think it was there from the early
days of soong when soong started at the top-level Android.bp file and
traversed downwards through references via the subdirs or
optional_subdirs variables.
Bug: 364749114
Test: Presubmits
Change-Id: I72eabe4d4179696bfa258c8c4c56351a132e0870
eda96ba50f