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
diff --git a/api/gen_combined_removed_dex.sh b/api/gen_combined_removed_dex.sh
index e0153f7..2860e2e 100755
--- a/api/gen_combined_removed_dex.sh
+++ b/api/gen_combined_removed_dex.sh
@@ -6,6 +6,6 @@
 
 # Convert each removed.txt to the "dex format" equivalent, and print all output.
 for f in "$@"; do
-    "$metalava_path" signature-to-dex "$f" "${tmp_dir}/tmp"
+    "$metalava_path" signature-to-dex "$f" --out "${tmp_dir}/tmp"
     cat "${tmp_dir}/tmp"
 done