Fix handling, documentation and testing of updateable flags.

Remove BIND_ABOVE_CLIENT from updateable flags since there
is not a clear use and its implementation is probelmatic.

Add additional documentation to encourage future authors to
validate changes to BIND_UPDATEABLE_FLAGS.

Make BindServiceFlags values accessible from tests.

Bug: 443357277
Flag: android.content.flags.enable_update_service_bindings
Test: atest android.app.cts.service.ServiceTest
Change-Id: I13fa20e87c67e49c0d20aed50c1b1e252c0161a8