libstagefright: Change CHECK in ExtendedCodec to conditional

Removes a metdata CHECK assertion of kKeyWMAVersion in ExtendedCodec
that can break playback if the key is not set. This only allows
overriding the WMA component name if the key in question exists.

Change-Id: I0eac4304d7fb9b38765ad002e48674c1b0c9acf9