Disallow autofill session for visible background users

Currently, autofill service does not fully support visible background users.
Prevent starting autofill sessions for visible background users to avoid issues with their autofill behavior.

Bug: 375500806
Test: manual - check that autofill is not working for a visible background user.
Flag: EXEMPT bugfix
(cherry picked from https://partner-android-review.googlesource.com/q/commit:9489b856d41b15df98a09e2c5e384e041892ade5)

Change-Id: Ibe368d7dd01d62ad5ca3e36ea3dd93e02077d7ad