[Autofill Framework] Add a debug log to help with triaging.

Currently when AutofillManager receives the view become invisible notification, it first checks whether client is visible (whether activity is stopped) before updating the visible/invisible tracked set.

However we don't have debug log printing out this event, which makes the issue hard to see.

Adding a debug log would make triaging the root cause b/333952473 much easier.

Bug:b/349185460
Test: atest CtsAutoFillServiceTestCases
Flag: EXEMPT bugfix

Change-Id: I1f271e2057d8cc24796578b3e36a6c9aceda3e10