Convert RuleBuilder to static rule in buildRuleToGenerateIndex

This saves ~34kb from the build.ninja file.

Test: Presubmits
Change-Id: I8fa4974e4f1509be7938c774e536d15a63534753

java/config/config.go

@@ -195,6 +195,7 @@ func init() {
 	pctx.HostBinToolVariable("ManifestMergerCmd", "manifest-merger")
 
 	pctx.HostBinToolVariable("Class2NonSdkList", "class2nonsdklist")
+	pctx.HostBinToolVariable("MergeCsvCommand", "merge_csv")
 	pctx.HostBinToolVariable("HiddenAPI", "hiddenapi")
 
 	hostBinToolVariableWithSdkToolsPrebuilt("Aapt2Cmd", "aapt2")