Change Terminal System App to use SYNCABLE_OS_PREF

Terminal settings are currently stored as SYNCABLE_PREF, but since
Terminal System App only relates to Chrome OS, I believe it should
be using SYNCABLE_OS_PREF.

Change-Id: Iead464a35f0bcbf2a39ca6958073f11ad385e43c
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2352252Reviewed-by: James Cook <jamescook@chromium.org>
Commit-Queue: Joel Hockey <joelhockey@chromium.org>
Cr-Commit-Position: refs/heads/master@{#797516}

chrome/browser/chromeos/crostini/crostini_pref_names.cc

@@ -124,7 +124,7 @@ void RegisterProfilePrefs(PrefRegistrySimple* registry) {
   registry->RegisterBooleanPref(kCrostiniDefaultContainerConfigured, false);
   registry->RegisterDictionaryPref(
       kCrostiniTerminalSettings, base::DictionaryValue(),
-      user_prefs::PrefRegistrySyncable::SYNCABLE_PREF);
+      user_prefs::PrefRegistrySyncable::SYNCABLE_OS_PREF);
 
   registry->RegisterIntegerPref(
       kCrostiniArcAdbSideloadingUserPref,