Sort hiddenapi monolithic files by signature

Adds a new --key_field option to merge_csv.py which specifies the name
of the field that should be used to sort the input. If specified it
causes that field to be the first in each row and performs the merge
operation of a merge sort on the input files. That assumes that each
input file is already sorted into the same order.

Modifies the rules that use merge_csv.py to pass in:
    --key_field signature
to sort the rows by signature.

Bug: 180387396
Test: Verified that hiddenapi files (both aggregated ones and for the
      individual modules) are not affected by this change other than
      changing the order.
Change-Id: Idcd5f0fea373b520b604889e1c280f21ed495660

java/hiddenapi_singleton.go

@@ -424,6 +424,7 @@ func metadataRule(ctx android.SingletonContext) android.Path {
 
 	rule.Command().
 		BuiltTool("merge_csv").
+		Flag("--key_field signature").
 		FlagWithOutput("--output=", outputPath).
 		Inputs(metadataCSV)
 
@@ -535,6 +536,7 @@ func (h *hiddenAPIIndexSingleton) GenerateBuildActions(ctx android.SingletonCont
 	rule := android.NewRuleBuilder(pctx, ctx)
 	rule.Command().
 		BuiltTool("merge_csv").
+		Flag("--key_field signature").
 		FlagWithArg("--header=", "signature,file,startline,startcol,endline,endcol,properties").
 		FlagWithOutput("--output=", hiddenAPISingletonPaths(ctx).index).
 		Inputs(indexes)