De-dup INTERNAL_SYSTEMIMAGE_FILES to fix PDK builds

On PDK builds, we sometimes end up with multiple entries in
INTERNAL_SYSTEMIMAGE_FILES, if we've got the definition for it, and it's
in PDK_FUSION_SYSIMG_FILES. This causes us to define multiple rules in
sdk_font.mk, leading to overriding commands errors.

Bug: 118634643
Test: attempt PDK build
Change-Id: Ieb4f46c0eece96cf246197af4b03e693493cc6a1