[DocsUI M3] Fix the grey background color on search chips row

CollapsingToolbarLayout has a default grey content scrim color which
will be incorrectly applied to search chips row sometimes, this CL
just set that color to transparent to avoid this. In Figma spec,
we don't want to show any content scrim color when scrolling.

Check the attached bug for the before/after comparison.

Bug: 414722239
Test: m DocumentsUIGoogle && manual inspection
Flag: com.android.documentsui.flags.use_material3
Change-Id: I663016e5b495d6dd9da7cde7c7395ebdd28a059c