[DocsUI M3] Update selection region circle with a fake shadow

Draw a 5% black circle with a white stroke. Simulate a shadow by
drawing 20% black ring at a 1dp offset from the circle.

See bug for screenshots.

Bug: 413744796
Test: m DocumentsUIGoogle && manual inspection
Flag: com.android.documentsui.flags.use_material3
Change-Id: Idf584b8f6f03d825311c4d86034b474a21f27640