DO NOT MERGE: osi: Offload mutex pointer to local scope

Create a shared_ptr for the callback_mutex in the alarm struct.
When performing the callback, make a local shared_ptr reference.
lock_guard on the local shared_ptr reference.

Bug: 117997080
Test: atest net_test_bluetooth
Change-Id: Iab800f720f4ccc4735e4d494e0d458eb97b40a4a