Clear Binder identity in AppTaskImpl#moveTaskTo

Not clearing it does not cause any problems now but to be uniform with
other methods in AppTaskImpl and to be future-proof, let's clear it
anyway.

Fix: 429332474
Flag: com.android.window.flags.enable_window_repositioning_api
Test: atest CtsWindowManagerDeviceOther:TaskMoveTests
Change-Id: Ib38e765a1b7ec0ea2a47c3c0de56d5fa5f3bbcfd