Prevent NPE in bt_gatt_callback

A thread setting bt_gatt_callbacks and
a thread using it may be different,
potentially causing an NPE if
bt_gatt_callbacks is accessed after being cleaned up.

Use a local variable to ensure safe access in threaded environments.

Bug: 353974484
Flag: EXEMPT, strict null check.
Test: atest BluetoothInstrumentaionTests
Change-Id: I2bd8dd62d0fab0e58db1c1b690406341fc251bf8