Do not apply letterbox when entering PiP

Because the design of animation always animate while the activity
is in fullscreen appearance, the PiP activity still needs to apply
letterbox when exiting PiP.

But when entering PiP, the condition is the same:
 task windowing mode=pinned
 activity windowing mode=fullscreen

So check mWaitForEnteringPinnedMode to exclude the entering case:
The order of applying WCT after the enter-pip animation is
"set PiP task bounds" -> "clear override of activity windowing mode".
The configuration change from "set PiP task bounds" will hit
the windowing mode condition as above.

This also replaces I83c5d32ef7cff4b3b6c85a698d068935c83c9423.

Fix: 283008030
Test: RootWindowContainerTests#testSingleActivityTaskEnterPip
Test: adb shell wm set-letterbox-style --cornerRadius 400
      An activity declares fixed orientation and requests to enter PiP.
      The PiP activity should not show weird round corner.
Change-Id: I22e294601b628adab9732bec373506c8543f116e