They do have the copyright to the code they wrote, but not to the code they took and changed (Apache Harmony, Linux kernel). They can relicense everything that originated from them, and provide it under a pure commercial license or dual-license it.

However, it might mean that they have to re-implement big parts of the Android runtime, or even do clean-room implementations by people not previously involved in those projects, to prevent legal attacks on the inevitably-similar code that results.