WindowContainer Overlays: Emit initial configuration change

Emit a configuration change event when attaching so that the remote end
has a change to process changes inbetween its initialization (where it
likely fetched a configuration from some third party) and attachment
times.

Bug: 213603716
Test: Existing tests pass
Change-Id: I053e4ed0210cf70e07d30fc6b10cf04ed30921d8