[Linux] Fix memory leaks in the BLE module (#28067)

* Do not duplicate string when passing to const argument

* Convert NULL to nullptr

* Do not duplicate string when adding to mpConnMap

* Free buffer used for reading characteristic data

* Clear stack-allocated dict variants

* Fix memory leak when creating unix FD list

* Change new to g_new to be consistent with the rest of the code

* Replace stack-alloc variant dict with heap-alloc

In our case, it's easier to handle memory free for heap-allocated variant dicts.

* Free memory returned by g_variant_dict_lookup_value