deprecate sharedUserId attribute

Shared user ids have caused numerous issues, inconsistencies,
and other random behaviours. They are generally not needed as
proper methodologies -- such as providers and services -- exist
and should be used for cross-package interaction.

We are deprecating the sharedUserId and sharedUserLabel
attributes in Q with the eventual goal of entirely removing them
in a future Android release.

Fixes: 129297713
Test: none. doc only change
Change-Id: Id84e8e22918010b2a42531c0b94aff75689d419b