AImageDecoder: Suppress log messages

Bug: 153071768
Test: AImageDecoderTest

For simplicity, I1aff544e8d6932b9ed0931a00da66a0aba6cd536 made
ImageDecoder skip specifying kOpaque when setting the SkImageInfo for
decoding. For some formats (e.g. jpeg and heif), this results in a log
message:

"Warning: an opaque image should be decoded as opaque - it is being
 decoded as non-opaque, which will draw slower"

This isn't relevant to AImageDecoder, which doesn't let you specify
kOpaque or not (since the intent is not to create an SkBitmap for Skia).

Now that ImageDecoder specifies kOpaque properly, the JNI code no longer
needs to correct it.

Replace another opacity check with ::opaque() for simplicity.

Change-Id: I7c270d9b9db61a61a338f40b056ce5d23a56e14d