Allow client (DocumentsUI) to specify search result limit

Searches of the FileSystemProvider subclasses (DownloadStorageProvider
and ExternalStorageProvider) are hard limited to return a maximum of
23 documents. This limit is not appropriate for all device types. This
change allows the DocumentsProvider client (the DocumentsUI app, which
is the system file picker and possibly system file manager) to specify
the result limit that it wants.

Bug: 414495650
Test: atest ExternalStorageProviderTest
Flag: com.android.internal.content.storage.flags.use_file_system_provider_search_limits
Change-Id: I5a9e45b49e076cce95d60eb35475c4788c9855af