Create a new group for foreground of multi-window

In multi-window mode, currently the focused window will be top-app,
and others will be foreground. However, the cpu resource we assign to
typical foreground group may not meet the performance need of those
windows. Therefore, create a new group for them so that we could
distinguish their performance need.

Bug: 200769420
Test: group created
Change-Id: I84c61f9ed4d167e156dafb7a816b6482e80a8523