[DocsUI M3] Centre items in grid layout

Previously items (columns) were not centred leading to any extra spacing
going to the righthand side.

Add an outer FrameLayout layer where width is match_parent which will
evenly distribute the space between items. Within the FrameLayout
centre the actual grid item with center_horizontal.

See bug for demo.

Bug: 404978773
Test: m DocumentsUIGoogle && manual inspection
Flag: com.android.documentsui.flags.use_material3
Change-Id: I379a9a97c0e6c3b8656a9c730e862cdc903de370