ControllerInterface: fix UpdateReferences() deadlock

Removed useless locks to DeviceContainer::m_devices_mutex, as they were all already protected by m_devices_population_mutex.
We have no interest in blocking other threads that were potentially reading devices at the same time so this seems fine.
This simplifies the code, and I've adjusted a few comments which mentioned possible deadlock that should now be totally gone.

The deadlock could have happen if a thread directly called EmulatedController::UpdateReferences(), while another another thread also reached EmulatedController::UpdateReferences() within a call to ControllerInterface::UpdateDevices(), as the mentioned function locked both the DeviceContainer::m_devices_mutex and s_get_state_mutex at the same time.

The deadlock was frequent on game emulation startup on Android, due to the UpdateReferences() call in InputConfig::LoadConfig() and the UI thread triggering calls to ControllerInterface::UpdateDevices().
It could also have happened on Desktop if a user pressed "Refresh Devices" manually in the UI while the input config was loading.

Also brought some UpdateReferences() comments and thread safety fixes from https://github.com/dolphin-emu/dolphin/pull/9489

Source/Core/InputCommon/ControllerEmu/ControllerEmu.cpp

@@ -18,6 +18,8 @@
 
 namespace ControllerEmu
 {
+// This should theoretically be per EmulatedController instance,
+// though no EmulatedController usually run in parallel, so it makes little difference
 static std::recursive_mutex s_get_state_mutex;
 
 std::string EmulatedController::GetDisplayName() const
@@ -77,6 +79,7 @@ void EmulatedController::UpdateSingleControlReference(const ControllerInterface&
 {
   ciface::ExpressionParser::ControlEnvironment env(devi, GetDefaultDevice(), m_expression_vars);
 
+  const auto lock = GetStateLock();
   ref->UpdateReference(env);
 
   env.CleanUnusedVariables();