Don't add FillEventHistory events to the wrong session.

The AutofillSession.getFillEventHistory() method returns only the event history
for the last onFillRequest(). In the scenario where the user switches
activities and the server has multiple sessions open, only the events for the
last session should be recorded.

Test: existing CtsAutoFillServiceTestCases pass
Test: LoginActivityTest.checkFillSelectionFromPreviousSessionIsDiscarded

Fixes: 62802026

Change-Id: I447ed77c2167095867b35d616b5cf2ae43aa28db