Fix BT keeping wrong state issue

When media session (such as music player app) is closed unexpectedly,
BT state is keeping wrong state even though there is no
music streaming.
This fixes this issue by adding a simple logic to check destruction
of media session then return null when requesting Playback state.

Bug: 125099920
Test: Test on phone and carkit as a remote node
Change-Id: I4eac1d2ad0848ad515f36e9352774d0fe5505aa5
Signed-off-by: Joonghong Park <joonghong.park@samsung.com>