Minimal Android Runtime APEX module.

Introduce build rules to generate com.android.runtime, a minimal
Android Runtime APEX module containing just ART and its dependencies.

When module com.android.runtime is built (`make com.android.runtime`),
make produces a `com.android.runtime.apex` package in the `apex`
directory of the system partition
(`$ANDROID_PRODUCT_OUT/system/apex/com.android.runtime.apex`).

This module is not built by default when generating a system image.

Test: make com.android.runtime
Test: art/build/apex/runtests.sh
Bug: 113373927
Change-Id: I2019bd1934558feba6eccef13b887b2faa96caec