Honor `PRODUCT_ART_TARGET_INCLUDE_DEBUG_BUILD` in Runtime APEX selection.

Include the Release Runtime APEX module on user builds and the "Debug"
Runtime APEX module on userdebug and eng builds, unless
`PRODUCT_ART_TARGET_INCLUDE_DEBUG_BUILD` is set. If so, and if this
variable is `true`, include the Release (smaller) Runtime APEX module;
if it is `false`, include the "Debug" (larger) Runtime APEX module.

This logic is similar to the one used for module `art-runtime`
regarding debug variants (also located in art/Android).

Test: make systemimage with user target.
Test: make systemimage with userdebug/eng target.
Bug: 113373927
Change-Id: Ib7f567a0d9910c4e60631565310369f383045076