DocsUI: let "single click on row" select an item

This changes mouse tap (click) behavior but not touch tap (or touch long
press) behavior.

Bug: 389814214
Flag: com.android.documentsui.flags.single_click_to_select
Test: atest DocumentsUIGoogleTests
Change-Id: If291cd1770d24f68ecf8a74601b8e8efbc7a952f