DocsUI M3: Try to fix InflateMessageDocumentHolderTest

According to the failing stack trace, it's failing when inflating a M3
resource item_doc_inflated_message_m3, but the theme's InheritanceMap
in the stack trace, shows using the legacy theme DocumentsDefaultTheme.

Fix by calling getRes() in the test to force the M3 theme in the setup.

Bug: 424456315
Test: atest InflateMessageDocumentHolderTest
Flag: com.android.documentsui.flags.use_material3
Change-Id: I4043954279bc94f9c49f98e614c950eb7c8775f4