Update AImageDecoder_getAlphaFlags return value

Bug: 135133301
Test: I2ea58bede5cf49a7947c96e7e6e37f967632ad6b

BAD_PARAMETER is a more sensible value than -1. The other return will
not be reached, but the code is clearer.

Change-Id: I4bee8f238f61a6c6cd13b5458f201f34d34315d9
1 file changed