releasetools: Move recovery-two-step.img to OTA/.

It used to be packed at IMAGES/recovery-two-step.img, but to serve OTA
purpose only.

Test: `m dist` with a non-A/B target. Check the file in the generated
      target_files.zip.
Test: Create two-step package. Check that recovery-two-step.img is used.
Change-Id: Iec6a73c682e0f844cd8c0b758c9470fa35dd15d8