Block offset requests prior to entering PiP

When entering PiP directly requested by
Activity#enterPictureInPictureMode in gesture nav mode, there is chance
that offsetPip is scheduled ahead of the actual entering PiP alpha
animation.

Fixed by migrating the mExitingPip and mInPip to a more meaningful
internal State enum and blocks resize requests till the animation is
kicked off.

Bug: 166084738
Test: See reproduce step in b/166084738#comment10
Change-Id: Ie78c9892c29423f10b001bb3d28549ac176cd1c3