Don't cancel animation if the previous insets hint was NONE

Before IME is visible, the insets hint of IME would be NONE. This
should not be considered as the side changing of a leash.

Fix: 387933048
Flag: EXEMPT bugfix
Test: atest ImeInsetsControllerTest#testChangeSizeWhileControlling
Test: Click the fullscreen button of a YouTube video in 3-button
      navigation mode, make sure nav bar is not visible during the
      rotation animation.
Change-Id: I7de1192da36c203708e54904948ec70cf2820b25