Ignore STOP command if A2DP stream is suspended

Ignore STOP command if A2DP stream is already suspended.
Some carkits send STOP command before VOIP call initiation
but don't send PLAY when call ends which hinders DUT to resume
audio playback after call termination.

Change-Id: I41cc1fc6f4b6cfaa1560d729baa0ca27f6dfe8c8