bp2build: fix BUILD file generation incrementality bug.

Before this change, changes to any Android.bp file would not trigger a re-run of bp2build/Soong, since the depfile for Soong's main output, the build.ninja file, didn't contain the list of Android.bp files. bp2build only reran when Soong itself changed, which happens regularly while developing bp2build, but not at all while modifying Android.bp files.

This change correctly dumps the list of all Android.bp files in the tree to the depfile for out/soong/build.ninja, therefore correctly triggering a re-run of bp2build when any Android.bp file changes.

Test: GENERATE_BAZEL_FILES=true m nothing; edit an Android.bp file; GENERATE_BAZEL_FILES=true m nothing -> successfully re-runs and regenerates out/soong/bp2build.

Test: inspect out/soong/build.ninja.d manually
Fixes: 179452534

Change-Id: Iad80c381b3afa38a4d4fafca035e46aa2a1f206a