Allow passthrough service manager to register client PIDs.

Test: lshal
Bug: 34712252
Change-Id: I54a615385f49b7bcb853811682d40e5b74c9acf7
diff --git a/HidlService.cpp b/HidlService.cpp
index e2fc93a..7e003d3 100644
--- a/HidlService.cpp
+++ b/HidlService.cpp
@@ -44,6 +44,14 @@
     }
 }
 
+void HidlService::registerPassthroughClient(pid_t pid) {
+    mPassthroughClients.insert(pid);
+}
+
+const std::set<pid_t> &HidlService::getPassthroughClients() const {
+    return mPassthroughClients;
+}
+
 std::string HidlService::string() const {
     std::stringstream ss;
     ss << mInterfaceName << "/" << mInstanceName;