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

java/hiddenapi_modular.go

@@ -1255,8 +1255,9 @@ func buildRuleToGenerateRemovedDexSignatures(ctx android.ModuleContext, suffix s
 	rule := android.NewRuleBuilder(pctx, ctx)
 	rule.Command().
 		BuiltTool("metalava").
+		Text("signature-to-dex").
 		Inputs(removedTxtFiles).
-		FlagWithOutput("--dex-api ", output)
+		FlagWithOutput("--out ", output)
 	rule.Build("modular-hiddenapi-removed-dex-signatures"+suffix, "modular hiddenapi removed dex signatures"+suffix)
 	return android.OptionalPathForPath(output)
 }