[DocsUI M3] Space grid items evenly

See the document "How to evenly space items dynamically with
GridLayoutManager" in the bug for an explanation.

Bug: 404978549
Test: m DocumentsUIGoogle && manual inspection
Flag: com.android.documentsui.flags.use_material3
Change-Id: Ib7a2ab8d6b1df2d5cf7d20893db3b2a5d697e4cc