Refactored ViewState.

- Added an int state.
- Removed mValueChanged and use state.
- Removed unused mAuth
- Set its Fillresponse in the proper places.
- Encapsulated private attributes.
- ...except mId (which is now id).
- Stored only id of current view state on Session.

This refactoring didn't modidy any behavior - in fact, the CTS tests didn't
change - but it will make it much easier to implement partitioning.

BUG: 35707731
Test: CtsAutoFillServiceTestCases pass

Change-Id: Ib07929a4089201a0e5bb66004af91f6cba362ba4