Surface tree statistics through metadata.

As part of storage privacy work in Q, we're trying to help users
understand the impact of OPEN_DOCUMENT_TREE choices, and the best way
is to show statistics about what's actually contained inside a tree.

Define "count" and "size" statistics, and populate them for all
FileSystemProvider instances.

Bug: 117975747
Test: atest android.provider.cts.DocumentsContractTest
Change-Id: Ib3f8f208c619141c26abaee0137641f12b009c8f