UDN Bug: ASTC compression becomes greyscale

Bug reported at https://udn.unrealengine.com/s/question/0D5QP00000tOpXN0A0/using-mutable-plugin-results-in-greyscale-textures-on-android-xr?fromCase=1 

• Setup your project so that it uses ASTC 6x6 block compression (Android ASTC, and have the ini setting DefaultASTCQualityBySize set to 3
• Cook and test on device to check that the color textures are not grayscale.
• Alternativelly, open in the Debugger tool from the Customizable Object Editor, and see the generated code compiled for the relevant platform.