Fixed placemente of Autofill UI for virtual views.

The PopupWindow anchoring offset should be based on the bottom of the anchor
view, but we were using top. Hence, the PopupWindow.tryFitVertical() was
failing to place it above the view in some cases, even when there was plenty
of space.

Fixes: 37991460
Test: manual verification with virtual view sample
Test: CtsAutoFillServiceTestCases pass

Change-Id: Iffdfe0d7858e865192338184d63bf5b93ba7108f