Extract SelectionManager interface.

This will be necessary to decouple selection manager from DocumentsUI.
Why? In order eliminate the "reset" method that DocumentsUI depends on
to accommodate it's fragment transaction model...without incuring
further DocumentsUI refactoring costs.
Eliminate refs to documetnsui.Shared.
Update tests to not use AndroidTestCase.

Change-Id: I56cc85c3659e2116203f829c6f0d46b32a6d3cb6
Bug: 64847011
Test: Small and medium passing.