Skip ForceDarkTests for visible background users.

Visible background users are not allowed to call
UiModeManager#setNightMode, therefore we cannot run ForceDarkTests for
visbile background users.
This change will not affect non-automotive form factors where visible
background users are not enabled.

Bug: 384879865
Flag: TEST_ONLY
Test: atest ForceDarkTests
Test: atest --user-type secondary_user_on_secondary_display ForceDarkTests
(cherry picked from https://googleplex-android-review.googlesource.com/q/commit:360c357e9e78fdee67da899234326ef308a7686e)
Merged-In: Ib2d5f8bd2bab4566fe05d863064d4ef4e665e339
Change-Id: Ib2d5f8bd2bab4566fe05d863064d4ef4e665e339