Update the timing of clearing SplitRequest

We clean splitRequest after enter split and use it as a flag to
do some function such as active split by child adjacent task if
split invisible.

However, in some cases, such as a task start another new task and
finish it immediately. In this case, split active finished and
split request reset but split still didn't invisible and new child
task appeared, and it trigger function mentioned above then cause
bug.

To avoid such case, we should update the timing of clearing
SplitRequest to cover trampoline tasks launch so it won't evict
child tasks unexpectedly while entering split.

Fix: 277556204
Test: manual
Test: pass existing tests
Change-Id: Ib3240c0b5037a3a20692c83e2daa8cd858125ca7