AImageDecoder: respond to API feedback

Bug: 186656488
Test: I4dc406e85709dda14af8b414e2da3d39bb07643b

AImageDecoderFrameInfo_getDuration returns
ANDROID_IMAGE_DECODER_BAD_PARAMETER on null info.

Clarify that AImageDecoder_getFrameInfo will succeed if
AImageDecoder_advanceFrame does, explaining why there are not more error
possibilities.

Explain more clearly what AImageDecoderFrameInfo_hasAlphaWithinBounds
means.

Change-Id: Ic02f1798904ad5ccc446fb4cd76cbff451a4e450