LeAudio: Fix removing pending operations on disconnect

The mock alarm class was adjusted for the flexibility
needed to detect the double alarm_free calls in the
modified volume control service test.

Bug: 335729656
Bug: 346707160
Test: atest net_test_gatt_conn_multiplexing bluetooth_vc_test
Flag: EXEMPT; Fix covered with unit tests
Change-Id: Ib46285b376ef9a49b4d01c0e5e1eedb2648eaff2