Add $(PRODUCT_OUT)/apex to the artifact path whitelist

$(PRODUCT_OUT)/apex is where shared libraries in APEXes get installed
into. The path can be considered as a fake path as the shared libraries
are installed there just to have symbol files for them under
$(PRODUCT_OUT)/symbols/apex for debugging purpose. Shared libraries in
APEXes are made available to the path at runtime by mounting APEXes via
apexd. The files under $(PRODUCT_OUT)/apex are never compiled into a
file system image (e.g. system.img)

Bug: 120846816
Test: build/soong/build_test.bash -dist -products mainline_system_arm64
Change-Id: I3727091214f08f945559fa5f0f1450e74c1a5c9b