Start a new session on manual request after session is "gone".

When the autofill service returns a null FillResponse, the session is marked
"gone" because the service cannot autofill it. But there might be cases where
the view structure change and it's now autofillable, so need to allow users
to manually request autofill again in such cases.

Fixes: 38205945
Test: CtsAutoFillServiceTestCases pass
Test: LoginActivityTest.testAutofillManuallyAfterServiceReturnedNoDatasets()
Test: LoginActivityTest.testAutofillManuallyAndSaveAfterServiceReturnedNoDatasets()

Change-Id: I9b23c255e563dd0646bf266d31ddb10dcc4f7f6d