using source path of aidl_hash_gen instead of output path

In the case of missing hash, aidl_hash_gen doesn't exist because of
build error. So provide source path instead.

Bug: 187841663
Test: run the command in error case
Change-Id: I5776c968bf4ac3062191c797c2ffe4aab10b2efd
diff --git a/build/aidl_api.go b/build/aidl_api.go
index 1f3e933..f452c8f 100644
--- a/build/aidl_api.go
+++ b/build/aidl_api.go
@@ -358,7 +358,8 @@
 				hashFile: android.ExistentPathForSource(ctx, hashFilePath),
 			}
 			if !dump.hashFile.Valid() {
-				cmd := fmt.Sprintf(`(croot && aidl_hash_gen %s %s %s)`, apiDir, versionForHashGen(ver), hashFilePath)
+				// We should show the source path of hash_gen because aidl_hash_gen cannot be built due to build error.
+				cmd := fmt.Sprintf(`(croot && system/tools/aidl/build/hash_gen.sh %s %s %s)`, apiDir, versionForHashGen(ver), hashFilePath)
 				ctx.ModuleErrorf("A frozen aidl_interface must have '.hash' file, but %s-V%s doesn't have it. Use the command below to generate hash.\n%s\n",
 					m.properties.BaseName, ver, cmd)
 			}