Set allowTouches after the fling animation end

Due to mIsUserInteracting in PipTouchState will be false after the
user drag window, the PiP window will not aute move when bounds
changed. Fix it by add a setAllowTouches call when fling animation
end.

Bug: 156431985
Test: Drag PiP window to bottom then show keyboard and check
Change-Id: Ib1dca59ee9511ced804876655e6bcb4938145343