[DocsUI] Temporarily disable testOpenInNewWindow_preservesFiles test

This test doesn't make sense on non-windowing devices (eg. phones) as
the user can't open a new window (which is where the test fails).

Disable the test to enable mainline train boarding today until we
properly gate it behind the appropriate conditions.

Flag: EXEMPT bugfix
Bug: 429356725
Test: atest DocumentsUIGoogleTests:com.android.documentsui.SidebarUiTest
Change-Id: Ieeb8a218ca2f3491ea6ab3094548daa316b31d87