Move adjacent tasks forward together to ensure occluding state

In multi-window split, if one of the adjacent task are moved to front,
we need to move another one forward to ensure all the other tasks will
be occluded by these adjacent tasks as expectedly. Add a flag to
distinguish whether to move the adjacent tasks together when setting
adjacent tasks.

Fix: 204130085
Test: atest TaskTests
Test: atest WMShellUnitTests
Change-Id: I34ccd2633b23425a978bd3df4acc50f17323de21