Refined session management so Save can be automatically called.

This is yet another big refactoring:

- AutoFillManager keeps track of its current AutoFillSession.
- Views call AFM.startSession(View) when they can trigger autofill.
  (virtual views can call it as well). At this point, the manager
  sets an AutoFillSession, gets the activity token, and passes it to
  the service.
- Subsequent calls to AFM.start() will be ignored since the session
  is set.
- When the Activity is gone, it calls AFM.finishSession().
- Simlilarly, virtual views could call it as well.
- Added getAutoFillValue() to View.
- Removed AFM.updateAutoFillInput(childId): virtual views should now
  call startSession(childId) to start a session, and use the
  VirtualViewListener callbacks for updates.
- Change AutoFillValue to use String (which is immutable) instead of
  CharSequence for text values.
- Check if view is enabled before auto-filling.
- Removed 'cmd autofill fill' since it would require the appCallback
- Automatically dismiss the snack bar after 30s
- Moved the "don't change autofill value when autofilling" Inception
  logic into the service side.
- Etc...

BUG: 34819567
BUG: 33269702
BUG: 31001899

Test: manual verification
Test: CtsAutoFillServiceTestCases passes

Change-Id: I5fad928d4d666701302049d142026a1efa7291cd
26 files changed