Update DirectoryAddonsAdapterPrivateSpaceTest 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 DirectoryAddonsAdapterPrivateSpaceTest
Flag: com.android.documentsui.flags.use_material3
Change-Id: Ib8aa4486f95336bb71c888eed262b4700b1caf20