Added new API to support multi-activity autofill workflows.

For example, when an app uses one activity to enter username, another for
password.

Bug: 112051762

Test: atest MultiScreenLoginTest # new API
Test: atest CtsAutoFillServiceTestCases # to make sure it doesn't break anything
Test: m -j update-api

Change-Id: Ia95888901ab72f75938efb87103cd76f5ec8ce09