## What changes were proposed in this pull request?
When parsing raw image data in ImageSchema.decode(), we use a [java.awt.Color](https://docs.oracle.com/javase/7/docs/api/java/awt/Color.html#Color(int)) constructor that sets alpha = 255, even for four-channel images (which may have different alpha values). This PR fixes this issue & adds a unit test to verify correctness of reading four-channel images.
## How was this patch tested?
Updates an existing unit test ("readImages pixel values test" in `ImageSchemaSuite`) to also verify correctness when reading a four-channel image.
Author: Sid Murching <sid.murching@databricks.com>
Closes#20389 from smurching/image-schema-bugfix.
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