Release Android Runtime APEX module.

Introduce a release Android Runtime APEX module
(`com.android.runtime.release`), containing only the essential parts
of the runtime (i.e. without debug variants nor tools). This lighter
Runtime APEX package is intended for user builds as well as for
storage-constrained devices (like marlin and sailfish).

Rename the original Android Runtime APEX module (which also contains
debug variants and tools) as `com.android.runtime.debug`, and include
it in userdebug and eng builds (except configured otherwise).

Also introduce phony package (build target) `com.android.runtime` as
an alias for module `com.android.runtime.release` (for now).

Test: make com.android.runtime
Test: make com.android.runtime.release
Test: make com.android.runtime.debug
Test: art/build/apex/runtests.sh
Bug: 113373927
Change-Id: I27a50aee9d83c8c3a9bd38636022e9f5335a2a0d