Use floating bar to show authentication sign-in request.

(A.K.A "R.I.P. temporary auto-fill notifications")

Also fixed scenario where flow switches back to the active from the
service authentication activity but service does not call FillCallback
again (for example, if user tapped back) - the UI should be disabled
in this case.

BUG: 33197203
Test: manual verification
Test: CtsAutoFillServiceTestCases passes

Change-Id: Ic7157ff70d2e6a7a25e83611b5eed3b6c80b55bd