Fix wrong condition used to calculate clearAllShortTotalTranslation

- We now calcualte the first task's start position, and compared against the expected position (mLastComputedTaskSize) to determine if we need extra translation for ClearAllButton
- The calculation is done with `mIsRtl = true`, and only only flip +/- when applying translation
- http://screen/brM3taew74Z3uZJ shows what happens without clearAllShortTotalTranslation:
  - Blue area is mLastComputedGridSize, green area is longRowWidth
  - firstTaskStart is caculated from left of grid size, adding longRowWidth
  - Red area is mLastComputedTaskSize, where focus task is in the middle (for grid-only-overview it's in top right)
  - clearAllShortTotalWidthTranslation is the yellow area

Fix: 354104098
Test: With normal/largest display size, LTR/RTL, focus task /grid only Overview, swipe up with 1/2/6/7 tasks
Flag: EXEMPT BUG_FIX
Change-Id: I324eebc0a2b2129a4ba62d4569bc555b74140c21