Use fixed width in overveiw grid

- Removed thumbnail ratio logic in TaskView, getVisibleThumbnailRatio will be clean up separately
- Removed quickswitch clipping and scaling, so overview and quickswitch looks identical. mClippedInsets will be clean up separately
- Put focused task in center of screen
- Refacotred CLearAllButton spacing

Bug: 187839470
Test: Manual
Change-Id: I2a7df1ac02acd5379af33f5b0e9abba0fab8e8d2