Remove `GroupTask.mSplitBounds`

See go/refactor-group-task for details. This CL removes `mSplitBounds`
from `GroupTask` and adds `splitBounds` to `SplitTask`. This makes its
usages more intentional.

Bug: 388593902
Test: m
Flag: EXEMPT pure refactor with no behavior change.
Change-Id: I9a3e8ee2e96d46f9eb79a58ffcf5efc257615aac