Additionally look for partition files under RADIO/ in target_files zip.

With this change, we will in turn look for partition image files under
IMAGES/ and RADIO/ in a given Android/Brillo target_files zip file.

We used to request the caller (e.g. ota_from_target_files.py) to have
all the updatable images ready under IMAGES/, by possibly copying some
of them from RADIO/ to IMAGES/. This CL avoids that cost, and keeps
working with legacy target_files zip files.

Bug: 77218220
Test: `m dist` produces the same full OTA package with the matching
      change to ota_from_target_files.py.
Test: It keeps working with legacy target_files zip that has everything
      under IMAGES/.
Change-Id: I9485508991115774ddbe70c1c235adf6806d91a4