Make DocumentCursor opt-out of having a self-observer

A content observer is registered by default when setting a notification
uri for a Cursor, in order to make the Cursor correctly notify listeners
of all changes to its URI, not just the ones made locally.

This is not required for DocumentCursor, because it already has a
separate mechanism for watching for all changes made to the data backed
by the cursor.

This avoids DocumentProviders having to call into system_server to
answer queries about directory trees, which can otherwise add up to
significant amounts of time for large directory trees. In my tests,
this improves the performance of iterating through a directory by
roughly 20%. This number is likely to be higher on non-test devices,
that probably see more binder contention, and will also depend on the
structure of the file tree.

Bug: 130276310
Test: SAF test app
Change-Id: I386363b0608c420e9847caf6fbf6686641c955e2