Apply extra round corners to first and last job item cards

This is done by setting the item view type depending on the position of
the item in the list. Then a different view can be created for the top
and bottom items.

Bug: 407673809
Test: atest -c 'DocumentsUIGoogleTests:com.android.documentsui.JobPanelUiTest'
Flag: com.android.documentsui.flags.visual_signals_ro
Change-Id: I63631cd225599c783c3c20e9b4be250ceb61aac0