[Autofill] Add DeviceConfig flags for Flags.multipleFillHistory

Adds DeviceConfig flag session_fill_event_history that will guard
against Flags.multipleFillHistory. This will allow us to do A/B
experiment of this feature in Android Beta.

Flag: android.service.autofill.multiple_fill_history
Bug: 384076035
Test: atest CtsAutoFillServiceTestCases
Change-Id: I6d0d875f3d5b602a871fb16000e5df3b2c59a6c2