Allow to finish session when all views are gone

An service can option to finish the session once all views that it
declared as important. Views that are important are all autofillable
views of any partition and the saveable fields of the last partition.

Test: CtsAutoFillServiceTestCases
Fixes: 35708237
Change-Id: I0ccade8ebb427e5d8928697ef0007c75d3f83df0