start HFP offload session when active device becomes not null

Previously, offload session only started when it is null. This causes issue when offload session is not null, but session is stopped. Then, it won't start session again

Bug: 348671524
Bug: 315234036
Flag: com::android::bluetooth::flags::is_sco_managed_by_audio
Test: atest net_test_bta
Change-Id: I8f3cc8d941bab0b0d917e9b1257d7e0aedded855