Let lock_guard deduce its template argument

No functional change, this is a cleanup.

With C++17, it's no longer necessary to specify the teplate argument
when it can be deduced from the types of constructor arguments. This
allows de-cluttering our locking statements.

To avoid typos, this patch was mechanically generated:

  perl -p -i -e 's/std::lock_guard<std::mutex>/std::lock_guard/g' \
    $(find . -name '*.cpp' -o -name '*.h')

Change-Id: Ibb15d9a6c5b1c861d81353e47d25474eb1d4c2df
diff --git a/server/NetdHwService.cpp b/server/NetdHwService.cpp
index 06723cf..d76075e 100644
--- a/server/NetdHwService.cpp
+++ b/server/NetdHwService.cpp
@@ -140,7 +140,7 @@
 }
 
 Return <StatusCode> NetdHwService::setIpForwardEnable(bool enable) {
-    std::lock_guard<std::mutex> _lock(gCtls->tetherCtrl.lock);
+    std::lock_guard _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) {
-    std::lock_guard<std::mutex> _lock(gCtls->tetherCtrl.lock);
+    std::lock_guard _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.