Handle page offset for grid overview

- Use TaskView's actual position with grid/fullscreen translation considered for taskPosition calculation
- Shift taskPosition by midpoint scroll, and no longer assumes midpoint is on middle of the screen
- Handle situation that TaskView is on left/right of midpoint, making the calculation generic to be able to handle grid situation

Bug: 175939487
Test: Launch modal view with wide/, RTL/non-RTL, orientation/simulated landscape combinations
Change-Id: Idd0cc9c5e24f453d830e1420319a38d3d784270d