Hides the Save UI while handling a pending intent from CustomDescription.

When the AutofillService sets a PendingIntent to launch an activity when
clicking a chield view (for example, to lauch a web page displayign the terms
and conditions of saving something), the system must hide the Save UI and
restore it after the new activity is dismissed.

That sounds simple in the surface, but it requires a huge refactoring behind
the scenes, such as injecting a token in the activity intent and using that
token to hide / cancel the UI during some activity lifecycle events.

Test: lotta of brand-new shinning tests on CtsAutoFillServiceTestCases
Test: cts-tradefed run commandAndExit cts-dev -m CtsAutoFillServiceTestCases

Change-Id: Ie8ec7aeb2c63cab68467046c1a9dcf85dbcc24ec
Fixes: 64309238