Exclude Assistant widget test

The "testAssistantWidget" test here:
https://source.corp.google.com/h/googleplex-android/platform/superproject/main/+/main:platform_testing/tests/automotive/functional/home/src/android/platform/tests/HomeTest.java;l=44 needs to be removed based on the `media_card_fullscreen` Flag. Since the test is in CatBox and flagging infrastructure is not supported there yet, the workaround is to exclude this test like in ag/26807253.

Bug: 331719655, 338476734
Test: atest --test-mapping
frameworks/base/packages/SystemUI:auto-end-to-end-postsubmit and make sure
testAssistantWidget test is not run.
Flag: NA

Change-Id: I5353f95e281fea76e276c988c6a8c8c7bae01c6b