[docsui] init test files via TestFilesRule in FileDeleteUiTest

FileDeleteUiTest was calling initTestFiles from the setup function, but
not cleaning up any leftover files since switching to the JUnit4 test
harness, because JUnit4 test harness assumes the reliance of
TestFilesRule and does not do any cleanup.

Bug: 407646510
Test: atest DocumentsUIGoogleTests:com.android.documentsui.FileDeleteUiTest
Flag: EXEMPT test change
Change-Id: Ie28e65dd24546c93fcf1436aea8cd8a0c96dc920