check error on setMediaCas

setMediaCas could fail even when the cas object is validated,
don't assume it always succeeds.

bug: 151006394

test: android.media.cts.MediaDrmClearkeyTest#testClearKeyPlaybackMpeg2ts
when the cfi check appears, mediaextractor will crash, and the
above test should show Fail result.

Change-Id: Ie4c04c5bd073e0000de893d16de466b36cb0fb86