Make hiddenapi flag generation use new artifact

Use the output of new genrule combined-removed-dex instead of the
removedDexApi output from various metalava runs when generating the
hiddenapi-flags.csv file.

There are some minor difference in the two combined-removed-dex files,
but these diffs do not amount to any diffs in the generated
hiddenapi-flags.csv file. See the full set of diffs here:
https://paste.googleplex.com/6632343525654528

Bug: 158465496
Test: diff hiddenapi-flags.csv before and after
Change-Id: I082d18fc2b8a57ea8c5941d5c955a7970ab9d860