Reduce some linear searches to cleanup the device registry (#112277)

Some of the data we had to search for was already available
in a dict or underlying data structure. Make it available
instead of having to build it every time.

There are more places these can be used, but I only did
the device registry cleanup for now

homeassistant/config_entries.py

@@ -1462,6 +1462,11 @@ class ConfigEntries:
         """Return entry with matching entry_id."""
         return self._entries.data.get(entry_id)
 
+    @callback
+    def async_entry_ids(self) -> list[str]:
+        """Return entry ids."""
+        return list(self._entries.data)
+
     @callback
     def async_entries(
         self,