Align the size calculation of task configuration

Since [1], screen dp size is rounded as px/density+0.5f.
So the task configuration should use the same calculation
for consistency. Which also avoids triggering unnecessary
1dp diff configuration change.

[1]: I8f78d9bd14b7fb9d12a20a662ad4ec04f82ce7f2

Bug: 204481252
Test: TaskTests#testComputeConfigResourceOverrides

Change-Id: I0fcd661551f38ce7f336b1319de5ae34826838a2