[DocsUI]: Fix failing test due to missing API.

Apparently toUnmodifiableList method is not available in JDK 11.
Switching to toList().

Bug: 428083510
Test: atest DocumentsUIUnitTests
Flag: com.android.documentsui.flags.use_material3
Change-Id: Ib783bc3fe1c70c83c7184ebcf5fa22878d7b5152