Adjust the display of job progress items

This adds two decorations to the recycler view - one to add a gap
between items, and the other to add rounded corners to the extremities
of the list.

Bug: 385841321
Test: manual inspection
Flag: com.android.documentsui.flags.visual_signals_ro
Change-Id: I8286f6e0beddaf7a3a2404393c4dbd1ddf29ce73