Replace RW lock with mutex, shared_mutex

Test: built, flashed, booted
      system/netd/tests/runtests.sh passes
Change-Id: I42b52d815b6ba0ba6f93dc27e83a900d2abec715
diff --git a/server/NetdHwService.cpp b/server/NetdHwService.cpp
index 65afe9b..06723cf 100644
--- a/server/NetdHwService.cpp
+++ b/server/NetdHwService.cpp
@@ -140,7 +140,7 @@
 }
 
 Return <StatusCode> NetdHwService::setIpForwardEnable(bool enable) {
-    android::RWLock::AutoWLock _lock(gCtls->tetherCtrl.lock);
+    std::lock_guard<std::mutex> _lock(gCtls->tetherCtrl.lock);
 
     bool success = enable ? gCtls->tetherCtrl.enableForwarding(FORWARDING_REQUESTER) :
                             gCtls->tetherCtrl.disableForwarding(FORWARDING_REQUESTER);
@@ -150,7 +150,7 @@
 
 Return <StatusCode> NetdHwService::setForwardingBetweenInterfaces(
         const hidl_string& inputIfName, const hidl_string& outputIfName, bool enable) {
-    android::RWLock::AutoWLock _lock(gCtls->tetherCtrl.lock);
+    std::lock_guard<std::mutex> _lock(gCtls->tetherCtrl.lock);
 
     // TODO: check that one interface is an OEM interface and the other is another OEM interface, an
     // IPsec interface or a dummy interface.