Use default dsu slot name when host is Android Q

DSU slotting support is added in Android R, so
android::gsi::GetActiveDsu() is guaranteed to fail if DSU guest system
is S and host is Q.
In that case, just assume the the DSU slot to be "dsu".

Bug: 172541102
Bug: 168571434
Test: Boot GSI S on Q pixel with DSU
Change-Id: I96e191907ce2fedf8c22b7887e930a7c455eb593