DocsUI: add a DirectoryListBot.selectDocument sleep

This test-only change adds a sleep after selecting a document. The
subsequent, already-existing assertSelection call already lead to a
wait(..., mTimeout) call, but an explicit sleep should make it clearer
that we need to allow the recyclerview-selection library to distinguish
between a (confirmed) single-click and a double-click, especially after
Iea74c19f158d60f55ba827c2ba53c623cbd293e8 (fix "double click on a
selected file") landed in the r-s library.

Bug: 432116371
Test: atest DocumentsUIGoogleTests:com.android.documentsui.ContextMenuUiTest
Flag: EXEMPT test change
Change-Id: Ibb8a8daa7a8e8c1616b115d6cc32f37d939d83df