Fix NPE in setActiveDevice if HFP audio is not allowed

Fix a null pointer expection that if a connected device is set to active
but audio is not allowed. If the previous active device is null, the
HeadsetService tries to send the null device to the native interface and
causes crash.

Tag: #stability
Bug: 201357034
Test: atest HeadsetServiceTest
Change-Id: I49f0b1cb690a7903a5b04813df4943382914866e