Destroy fill UI when app dies.

This is a regression caused by the autofill compat feature change, as the
Fill UI window was changed from a sub-panel which is auto managed by the window
manager to an overlay, to ensure the fill UI covers all app windows, so compat
mode would work for apps without app changes.

Test: atest CtsAutoFillServiceTestCases:SessionLifecycleTest#testDatasetGoesAwayWhenAutofilledAppIsKilled
Test: atest CtsAutoFillServiceTestCases:SessionLifecycleTest#testSaveRemainsWhenAutofilledAppIsKilled
Test: atest CtsAutoFillServiceTestCases # usual flakiness
Test: manual verification by adb shell kiling an app while UI is shown

Fixes: 73566982

Change-Id: I42f0acbaf3d0b19c081b8cb3613bebb01ceb487a