Fine tune tests

1. Add DialogFragmentTest removed tests back.
2. Change its location to functional folder.
3. Rename it as DialogUiTest.
4. Set it as LargeTest.
5. Remove testOverlayPackagesForDocumentsUI_shouldBeMutable because
   isMutable field cannot find in mainline apk.

Bug: 150257481
Test: atest DocumentsUIGoogleTests
Change-Id: I82d42e56834cf49c1da557cd12dcc152e8b15924