bluetooth: tbs: fix issues for setting URI scheme for GTBS

- "bt_tbs_set_uri_scheme_list()" function does not setting URI for
 GTBS instance. Using "inst_lookup_index()" fixes issue to find
 registered GTBS instance for provided bearer_index.
- "bt_tbs_set_uri_scheme_list()" function overrides URI scheme list
 with latest provided list. But the expectation is that it should
 keep previously added URI schemes and only update new scheme which
 are not part existing list. Code is updated to fix this.
- The same function uses net-buf to combine all URI schems for all
 TBS bearer if current request is not for GTBS instance. Currently,
 the code only uses combined URI scheme list for logging purpose and
 it is not used to send for URI Scheme notification to provide
 updated GTBS URI scheme to remote. Code is now updated to add
 dynamic buffer to add all TBS URI scheme list for GTBS instance
 and notify it similar to other functions to optimise memory usage.

Signed-off-by: Nirav Agrawal <nirav.agrawal@nxp.com>