libstagefright: Avoid flush if LPA playback is not routed

-On a previously closed LPA playback due to pausetimeout, if
reset is called flush is issued on closed sink leading to crash.
-Check if audio is routed before calling flush to avoid such
issues.

Change-Id: Iec24d063425b0a0017adcf0b3a576fbea0d8ef25