AudioService: fix A2DP disconnection / reconnection

Fix issue introduced by commit d32a94d2 where in case of an
A2DP disconnection, the device was disconnected then reconnected.

Bug: 146544156
Test: disconnect A2DP device, check it's not listed in dumpsys media.audio_policy
Change-Id: Ic5f9387194d92a7a050e7a295a13a9ae18cd96ce