Refactor AutoFill server code.

With this change, data flows strictly like so:

AFManagerService -> AFManagerServiceImpl -> Session -> ViewSession

A ViewSession is introduced to 1) know when we can show the fill UI,
and 2) know when we need to create or update the current fill UI.

Each class has its own responsibilities:

* AFManagerService - interface to app process
* AFManagerServiceImpl - per user id
* Session - per activity
** AutoFillUI - per activity (can consider moving to ViewSession)
* ViewSession - per view

Bug: 34633695
Change-Id: Ia197a5c40219379ab6a95d0cf5c39ac4f5470d0e
Test: CTS, Manual with GMS Core