AM: fix bug in moveTaskToBackLocked due to LockTask

In the previous refactor of LockTask-related methods, there was a typo
which resulted in an incorrect lock task state being reported during
moveTaskToBackLocked.

Bug: 66224859
Test: cts/tests/framework/base/activitymanager/util/run-test CtsActivityManagerDeviceTestCases android.server.am.ActivityManagerPinnedStackTests
Test: bit FrameworksServicesTests:com.android.server.am.LockTaskControllerTest
Change-Id: Ic235069ed3c2bafb14efcd83b98af12e37fcb5a7