Fix unbind service error for MusicFxHelper

Unbinding an unbound service will cause an exception,
for example: service not registered: com.android.server.audio.MusicFxHelper.

Flag: EXEMPT bugfix
Bug: 332454067
Bug: 350408926
Test: run MTBF test and no crash
Test: MusicFx panel enable/disable
Test: atest com.android.server.audio.MusicFxHelperTest

Change-Id: Ic519221b2bb7d28e5378a995519f8b3269d03c70
Merged-In: Ic519221b2bb7d28e5378a995519f8b3269d03c70