Create variants for each image type

Create variant of image, zip or flattened according to
TARGET_FLATTEN_APEX and payload type.
If payload type is zip, only zip variant is created because flattened
apex is not supported. And if payload type is image, image and flattened
variants are created.

Bug: 139053989
Test: m -j
Change-Id: Ibde18490d23ec602c4cca97cf97db90a562e014e