Remove a workaround for accessibility managers that works across users.

The system and some processes such as the keyguard and system UI
run across users. Hence, the local accessibility manager should call
into the backing system service with the id of the current user.
For all other processes the local manager uses the current user id.
There was a method that is to be called before the local accessibility
manager has been accessed to initialize it to work across users.
This had to be done for keyguard and system UI.

This change removed the workaround and now the local accessibility
manager determines under the hood which user id to use while calling
into the system. If the local manager is in the system process or
its process has permissions to work across uses, the manager uses
UserHandle.USER_CURRENT, otherwise it uses the user if its process.

Change-Id: I3e97fe98805adca01c1a0127a276828e90926f95