Introduce an aconfig flag to enable dream overlay host.

Currently, a dream overlay is attached to the dream's window. This
flag enables a feature that introduces a "host" that can display the
overlay without needing the dream's window (useful in those cases where
the dream is windowless, for example).

Test: m android.service.dreams.flags-aconfig-java
Bug: 291990564
Flag: NA
Change-Id: Idad0a4ce791651a5839d0d2bb3187902b4e75c0b