Implement MTU changed callback for GATT server

Implement MTU changed callback for GATT server

[Model] All
[BinType] AP
[Customer] OPEN

[Issue#] N/A
[Request] Internal
[Occurrence Version] N/A

[Problem] Unable to notice MTU changed event to application
[Cause & Measure] Implement MTU changed callback for GATT server
[Checking Method] n/a

[Team] Convergence BT
[Developer] Injun Yang
[Solution company] Samsung
[Change Type] Specification change

Change-Id: I57a539cadf7ab83931a40eaed573de9f9cecbd9c
Signed-off-by: injun.yang <injun.yang@samsung.com>
Signed-off-by: DoHyun Pyun <dh79.pyun@samsung.com>