Evict all main stage tasks only for non-drag split starts

* See bug for root cause info.
* Technically we'd want to expand the scope of this
to not only drag n drop but all explicit split starts, but
for QPR2 we'll keep it contained.
* The reason we evict has to do w/ launch-adjacent split
scenarios (highlighted in original bug b/279720005)

Test: Original bug doesn't repro nor does the current one with
drag and drop. Tested Chrome w/ some multi-instance drags and
that also seems to work fine
Fixes: 309905087
Flag: None

Change-Id: I3b4238141d8fadf0cde2ea7fe9fe1161496ce888