kleaf: Fix KBUILD_MIXED_TREE for 6.13. When KLEAF_INTERNAL_KBUILD_MIXED_TREE_IS_OBJTREE is set (on 6.13 branch), do not create KBUILD_MIXED_TREE; instead copy all GKI files to OUT_DIR beforehand. Bug: 402231876 Change-Id: If4eb07ec002cba934ec8ef468ce8d6c0b1b09a65