Fix getOrientation from adjacent TaskFragments

Before, we returned SCREEN_ORIENTATION_UNSET from TaskFragments in
split. Now, we returned SCREEN_ORIENTATION_UNSPECIFIED just like system
split screen, so that the display can update to device orientation after
exiting a fullscreen fixed-orientation activity to ActivityEmbedding
split.

Bug: 255646506
Test: atest WmTests:TaskFragmentTest
Change-Id: Iee97547aa5697df050b1e01070d9bad643325f14