[DocsUI M3] Update cross profile header in nav tree

When picker was triggered from the work profile, both personal
and work roots will show in the navigation tree, this CL updates
the header style.

Check the attached bug for comparison.

Bug: 408925447
Test: m DocumentsUIGoogle && manual inspection
Flag: com.android.documentsui.flags.use_material3
Change-Id: I5be9eebec6b2677e00cd17be63e41e16f3428521