Fix black screen after moving top activity to another display

The screen was black after moving the top activity to another display.
The next top activity did not be resumed and was still invisible. The
regression was from bebd8cd9, which incidentally reverse the condition.

Since the case of removing a display is already handled by 402b440f,
#postReparent is not needed to call again in #reparent (when display
is removing).

Fixing the issue forward by resuming the focusable task in
#moveRootTaskToTaskDisplayArea.

Bug: 192616155
Test: atest ActivityTaskManagerServiceTests
Change-Id: Id0971ad8a1c692fdb5376e2c077faa6f00dd61ac