Regression in screen introspection APIs due to the multi-user change.

1. The initial user was set to USER_NULL but some clients were registering
   before the user change callback happens. Since the initial user is
   the owner the current user id defaults to USER_OWNER.

2. The check for global clients and window connections was using the
   calling UID but there are processes that run in a per user basis
   as system UID (Setting for example). Now the check is stronger
   and comparing the caller PID with that of the system process.

3. The code for finding the focused window id was not checking the
   global window token list in addition to that of the current user.

4. The code updating the active window id was calling out into the
   window manager with a lock held.

bug:7224670

Change-Id: I9f4b7ea67eb5598b30ee7d1b68a1d3ce0cf8cfb4