stagefright: Fix fallback mechanism to SW decoder

Fallback mediaserver to SW decoder breaks
with commit eeb8a901

Since the err parameters is overrided in a local loop.
And even an alternative decoder is configureCodec
sucessfully, the outer loop err parameters is still != OK
and return false.

Change-Id: Id5a9e570e5f55f28a80d7cd25a2a14d59442f3e2