Add subdir to GenPath

We were emulating this for proto files, standardize it and make the
other generators use it as well.

Test: Compare out/soong/build.ninja before/after change
Test: mmma -j system/tools/hidl
Change-Id: I1888c7b981749060a398387bbb9b481270bf6d75

genrule/genrule.go

@@ -196,7 +196,7 @@ func GenSrcsFactory() (blueprint.Module, []interface{}) {
 		for _, in := range srcFiles {
 			tasks = append(tasks, generateTask{
 				in:  android.Paths{in},
-				out: android.WritablePaths{android.GenPathWithExt(ctx, in, properties.Output_extension)},
+				out: android.WritablePaths{android.GenPathWithExt(ctx, "", in, properties.Output_extension)},
 			})
 		}
 		return tasks