Let view focus move across adjacent task fragments

This CL moves the focus to the adjacent task/activity/window
(if there is any) while the next focused view cannot be found with the
given direction in the local window.

Fix: 283618824
Test: atest TaskFragmentTest#testMoveFocusToAdjacentWindow
Test: Run commands: `adb shell input keyevent KEYCODE_DPAD_XXX` while
      running 2-pane Settings.
Change-Id: I45681150e9edd25d59136c5762586c504cb301f9