Remove ENABLE_OVERVIEW_GRID feature flag

- Use isTablet directly instead of overviewShowAsGrid
- Tablet/landscape resoruces can now be overriden via xml
- Use Landscape resources in tablet in additional to vertical bar layout
  - For resources overwritten in values-land that was intended for vertical bar layout, override them in sw600dp for tablet landscape

Bug: 210597891
Test: manual
Change-Id: I19eac935a5fb1a420485408aa8b4774632726af1