Fix null pointer exception on AvrcpTargetService

This fixes null pointer exception by simply adding null pointer checker.
Problem occurs when sendMediaUpdate() is called just after Avrcp
Service is stop()ed.

Error Message: java.lang.NullPointerException: Attempt to invoke virtual method ’void com.android.bluetooth.avrcp.AvrcpNativeInterface.sendMediaUpdate(boolean, boolean, boolean)’ on a null object reference
Test: Once issue

Change-Id: Id42e48faa28ce6c1ec6d4148705d3342261aa6ca
Signed-off-by: slki.shin <slki.shin@samsung.com>