Add new quickstep test for many tasks open at once.

This test is needed for testing overview-grid-related functionality.
- Opening non-focused tasks
- Dismissing a non-focused task from the grid
- Grid rows rebalancing after dismissal, which only happens when enough
apps are open to be off-screen.
- Grid tasks do not scroll after dismissal.

Test: TaplTestsQuickstep.java
Bug: 197630182
Change-Id: Ic907db4643cdc2eb9e4610dab917347e234e470c