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
diff --git a/core/Makefile b/core/Makefile
index 6721757..55bcbcc 100644
--- a/core/Makefile
+++ b/core/Makefile
@@ -1784,12 +1784,12 @@
     $(filter-out $(foreach p,$(overridden_packages),$(p) %/$(p).apk), \
         $(ALL_PDK_FUSION_FILES))
 
-INTERNAL_SYSTEMIMAGE_FILES := $(filter $(TARGET_OUT)/%, \
+INTERNAL_SYSTEMIMAGE_FILES := $(sort $(filter $(TARGET_OUT)/%, \
     $(ALL_GENERATED_SOURCES) \
     $(ALL_DEFAULT_INSTALLED_MODULES) \
     $(PDK_FUSION_SYSIMG_FILES) \
     $(RECOVERY_RESOURCE_ZIP)) \
-    $(PDK_FUSION_SYMLINK_STAMP)
+    $(PDK_FUSION_SYMLINK_STAMP))
 
 FULL_SYSTEMIMAGE_DEPS := $(INTERNAL_SYSTEMIMAGE_FILES) $(INTERNAL_USERIMAGES_DEPS)