Make getTaskSize actaully a getter that does not modify member variables

- This is a follow-up of ag/26255150 to clean-up usage of getTaskSize
- Removed getTasksize and updatePivots in onLayout, as updateSizeAndPaddings has covered all scenarios
- mLastComputedTaskSize will only be updated from updateSizeAndPaddings, with minor refactoring on how mTaskWidh/mTaskHeight/padding are calculated

Bug: 325314248
Test: Manual on home/app->overview, rotations, fold/unfold etc.
Flag: None
Change-Id: I031a36e5e065a28070fb3f7f146978279f7d8886