Add 'use_debug_art' to product_variable

Add 'use_debug_art' property to product_variable so it can be checked
from Android.bp whether ART debug APEX or ART APEX should be used.

Bug: 351698657
Test: AOSP CF eng build succeeded
Test: AOSP CF userdebug build succeeded
Change-Id: I01cd80f0d899c55828ecb05fdafdc9bb05fd8200