Include fonts as non asset file for CorePerfTests

Android.bp migration broke this as the external folder should no longer
be referenced directly.

See b/169071405 for request for assets_dir filegroup support to avoid
openNonAsset.

Bug: 168845570

Test: atest android.graphics.perftests.TypefaceCreatePerfTest

Change-Id: If1ee2cadbff02ac85c60dea49de50da2e98c1b39