[DocsUI Peek] MaterialTextView and background corners

Use MaterialTextView instead of TextView, MaterialButton instead of
Button, using the fully qualified name explicitly instead of relying
on the theme inflator.

Define background shape drawables for rounded corners instead of
programmatically set a MaterialShapeDrawable as background.

Bug: 382163595
Flag: com.android.documentsui.flags.use_material3
Flag: com.android.documentsui.flags.use_peek_preview_ro
Test: Visually verified that the UI is not impacted by this change.

Change-Id: I77c150f1233e967265b66bf2eab1a8647fafdc20