Clear mSeamlessRotating in InsetsSourceProvider.setWindow

Otherwise, if the insets source window is changed during seamless
rotating, mSeamlessRotating will leak, which makes the insets cannot be
controlled anymore.

Fix: 193277149
Test: Open a landscape app which hides navigation bars from the portrait
      lockscreen on a device with taskbar. See if taskbar can be hidden.
Change-Id: I8d2218d2da44cb09f0488ac6d00352e2900665e7