Update DirectoryAddonsAdapterTest for use_material3

There is no break between files and folders when the use_material3 flag
is enabled, this changes the expectation in 2 of the tests where they
add 1 to the expected item count due to the artificial item injected to
break the layout.

Bug: 412776499
Test: atest com.android.documentsui.dirlist.DirectoryAddonsAdapterTest
Flag: com.android.documentsui.flags.use_material3
Change-Id: I1a7076fafaf16e3d4771a4aefa8996ba147af1fb