libmedia: Initialize state vars to avoid possible crash

If playback or record track creation fails on server side,
some state is left initialized incorrectly.
This can cause a crash when the object is destroyed
because of a reference to a NULL object.

Initialize state vars properly in the initialization list
to avoid this behavior.

authored-by: Zhou Song <zhous@codeaurora.org>

CRs-Fixed: 992608
Change-Id: If8c3611e3229c0c1b14b81285e07b9357fec7658