Remove fullscreen asserts for keyguard tests in ActivityVisibilityTests.

Whether a task remains freeform over keyguard or goes fullscreen is up to presentation/UX, not a WM policy.

Bug: 348699635
Test: ActivityVisibilityTests
Flag: TEST_ONLY
(cherry picked from https://android-review.googlesource.com/q/commit:2be41a0ea25cfb67c7f2b0edcff97e7db342617f)
Merged-In: Ia588c34315a3ad4defbd48e8cfc89d8b020fef43
Change-Id: Ia588c34315a3ad4defbd48e8cfc89d8b020fef43