Weaken UnpackJobTest

Since the underlying DocumentProvider might report an incorrect size for
extracted files, we ignore the reported sizes and we simply test for the
presence of extracted files.

Bug: 429262410
Test: atest DocumentsUIGoogleTests:com.android.documentsui.services.UnpackJobTest
Flag: com.android.documentsui.flags.use_material3
Flag: com.android.documentsui.flags.zip_ng_ro
Change-Id: Ief34d344d9c5975b5fb057d5abcc97d65eaf97e1