No longer follow subdirs in Android.bp files

because there some symlinks named Android.bp that point to
files named Android.bp that contain <subdir> entries that
are supposed to be interpreted relative to the original
symlink. It's more convenient to just skip following subdirs.

Bug: 64363847
Test: make -j
Change-Id: Ib15f1d1c7d511191fd841b1a29ceed559dd6cdbb