Rename TaskStackContainers to TaskContainers (85/n)

All stacks are now tasks and there will be no stacks soon.
CL helps reduce the size of future CLs to remove stacks.
Also:
- Removed getSplitScreenPrimaryStackIgnoreVisiblity and have callers
use getSplitScreenPrimaryStack instead.
- Removed getRecentStack() that is only used during dump().

Bug: 80414790
Test: Existing tests pass.
Change-Id: Ic9c6ea02c14a5508771c6513f187eff6b4b61e90