libcameraservice: Allow media server to disconnect camera even unlocked

The camera service may fail to release the camera hardware instance
in some use cases.
When an application unlocked the camera before disconnect, disconnect
from the application will not be accepted. And disconnect from media
server will not be accepted also. Then, the camera hardware instance
will not be released and a resource leak will be caused.

Allow media server to disconnect the camera at all times even if
the camera is unlocked.

Change-Id: Icd5ed81bed242fa5947aa40ca85e4ca7fa7286e7