Set PRIVATE_TARGET_AAPT_CHARACTERISTICS even when running with the old aapt

In commit e758f9319dcb6cc9693af03cc03d385167a35a8d we switched from
TARGET_AAPT_CHARACTERISTICS to PRIVATE_TARGET_AAPT_CHARACTERISTICS in
add-assets-to-package but forgot to set the variable when we are running
the old aapt.

Bug: 27874886
Change-Id: I03ced3030dcb9f024eef5b23dbfd5d99157952ea
2 files changed