DO NOT MERGE: Consolidate queryChildDocumentsXxx() implementations

Make sure to override the single right variant of the
FileSystemProvider#queryChildDocuments() method: the one that takes the
"includeHidden" boolean argument.

Bug: 200034476
Bug: 220066255
Bug: 283962634
Test: make, install and run manually
(cherry picked from https://googleplex-android-review.googlesource.com/q/commit:94678ecc096f91e5e5e96e58a31260e06711720a)
Merged-In: I4c00693e28f3d50d716350a65e9e6bfd7482b085
Change-Id: I4c00693e28f3d50d716350a65e9e6bfd7482b085