Continue pausing after entering pip2

Make sure we schedulePauseActivity after
moving the pip activity to a pinned root task.

Recall that moveActivityToPinnedRootTask is getting
called after PipTransition#handleRequest() in PiP2
since the entry bounds are needed in startWct.

Bug: 315532935
Test: atest CtsWindowManagerDeviceActivity:PinnedStackTests\
        #testAutoEnterPipIsInPictureInPictureModeInOnPause
Test: manually auto-enter PiP in 3-btn-nav
Change-Id: I8d754403f895582dacb41e43846c51b273a6f032