Use metalava's signature-to-dex subcommand

Use the new `signature-to-dex` subcommand rather than the deprecated
`--dex-api` option. The subcommand is already used elsewhere and uses
basically the same code under the covers as `--dex-api`.

Bug: 307737035
Test: TH
Change-Id: I649c5eccb332c6d0993c890714245040de37540e