Finishing root activity if having other running activities

When split activity, the back event can be sent to the root
activity while still having the embedded activity displaying
on top of the task. In that case, the task shouldn't be moved
to back.

Bug: 189385926
Test: finish root activity while split activities in task
Change-Id: If85a10928a40cd15f60e88f313c09d0481788ea8