Keep selection of widget at the end of widget dragging

Previously, the Glanceable Hub clears widget selection as soon as a
widget dragging starts. This change makes it so that a drag action also
selects that widget being dragged, and does not reset it after the
dragging completes.

Test: atest CommunalEditModeViewModelTest
Test: verified selection remains at the end of a widget dragging;
      see bug for before and after video
Fix: 376492600
Flag: com.android.systemui.communal_hub
Change-Id: I51970469cb73c77e909142b154cbbc67f4ae4dad