Allow PRODUCT_COPY_FILES to copy raw APKs into system_other

Files in this partition aren't actually installed, so we don't need to
use the BUILD_PREBUILT logic for APKs.

Bug: 66960479
Test: try PRODUCT_COPY_FILES of an apk into system, errors
Test: try PRODUCT_COPY_FILES of an apk into system_other, accepted
Change-Id: Ie721d2b884c6badfbe0e46f55c265b770bed618b