Migrate to use new signature-to-dex --out option

In order to support multiple input signature files `signature-to-dex`
was changed to take the output file as an option not the second
argument. This change switches the script to use it.

Bug: 307737035
Test: m out/soong/hiddenapi/hiddenapi-flags.csv
Change-Id: I2251266edffa4eae59630afbd407c28597c209d7