Initial IME integration.

- Created an AutoFillManager class, which provides methods to show
  the auto-fill bar for views and virtual nodes.
- Automatically launches an auto-fill request when the IME is shown
(and an AutoFillService is set for the given user) on TextViews.
- Updated VirtualNodeListener to use this new API.

BUG: 31001899
BUG: 34171325
Test: CtsAutoFillServiceTestCases passes
Test: manual verification

Change-Id: Id72ce97da70217081b5823cfc7b138412634fcf3