Minor cleanups to SelectionManager interface.

This is a small grab-bag of fixes:
- Address comments from ag/2768118
- Address comments from ag/2758092
- Address comments from ag/2767710
Rename the selection collector method from getSelection(Selection) to copySelection.
    + Don't return the selection.
Improve docs in some locations and drop TODOs for interface-smells
    in need of further attention.

Bug: 64847011
Change-Id: Ibe4038ed7fcd4bc5dd0bec3a8ada5f46b77932b7
Test: Passing.