Add module `libopenjdk` to the Android Runtime APEX module.

This is so that we can remove it from `PRODUCT_PACKAGES`, i.e. no
longer install it on /system, as module `libopenjdk` depends on module
`libart`; the installation of the former would trigger the
installation of the latter, which we are trying to stop (see companion
change in project platform/build).

Also label `libopenjdkd` as a Libcore library in the Runtime APEX
definition.

Test: art/build/apex/runtests.sh
Test: Device boot test with Android Runtime APEX.
Bug: 113373927
Change-Id: I17a6dec4b7b4e813a6d55c4aafbad2e793bfd69d