Use percentage scaling for overview thumbnail size

- Limit overview to screen ratio to be at max 70%
- Tuned row spacing and page spacing
- Added 2 more test activities to testOverviewForTablet to avoid dismissing tasks at end of grid, as more grid tasks are now shown

Bug: 207350366
Test: manual
Change-Id: Id04871464bdb3ef2d6b046977b0ffd9bd51bc230
Merged-In: Id04871464bdb3ef2d6b046977b0ffd9bd51bc230