Add AImageDecoder_resultToString

Bug: 160984428
Test: I118eaa99cd65b70dbfb2c8a7731c714564911a34

This allows clients to print a readable string when there is an error
(e.g. "ANDROID_IMAGE_DECODER_BAD_PARAMETER" instead of "-5").

Change-Id: I24400979e1ce12d5c8b48bba456b07fdfda49662