ambient: watch prefs if ambient is allowed for user account

Previously monitored prefs only if ambient mode was already enabled.

BUG=b:169579102

Cq-Include-Trybots: luci.chrome.try:linux-chromeos-chrome
Change-Id: I287ab1e710b8c45b6fb2b90ba6879fb5726fc2f5
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2535111
Commit-Queue: Jeffrey Young <cowmoo@chromium.org>
Reviewed-by: Xiaohui Chen <xiaohuic@chromium.org>
Reviewed-by: Tao Wu <wutao@chromium.org>
Cr-Commit-Position: refs/heads/master@{#827863}

ash/ambient/ambient_controller.cc

@@ -343,8 +343,11 @@ void AmbientController::OnFirstSessionStarted() {
 
 void AmbientController::OnActiveUserPrefServiceChanged(
     PrefService* pref_service) {
-  if (!IsAmbientModeEnabled() || GetPrimaryUserPrefService() != pref_service)
+  // TODO(b/170510846) make ambient controller handle pref off state better
+  if (!AmbientClient::Get()->IsAmbientModeAllowed() ||
+      GetPrimaryUserPrefService() != pref_service) {
     return;
+  }
 
   pref_change_registrar_ = std::make_unique<PrefChangeRegistrar>();
   pref_change_registrar_->Init(pref_service);