Fix positioning of split instruction and share targets with taskbar in overview

- When taskbar is in overview, overview actions has dedicated space and no need to align to nav buttons
- Add a bottom margin to split instruction and share targetse that is equal to the space below overview actions

Bug: 245320601
Test: Test split select and share target with different screen sizes
Change-Id: I6aec325da9cc4c43c84ba8bed5f0e087c57d6925