stagefright: ACodec: Don't return success when encoders fail to init

When configureCodec fails we try to fallback to another codec that
supports the mimeType. However, this fallback is only done for
decoders and, if encoders fail, they will always end up sending
a success notification and returning true, causing mediaserver to
crash.

Change-Id: I32bee3167a8b43243cd708258435174f90f21a1b