Fixed index out of bounds issue when removing windows.

Child windows are also removed when WMS.removeWindowInnerLocked()
is called to removed a window. This causes the number of windows
to decrement by more than 1 which causes an out of bounds exception
in AppWindowToken.removeAllWindows() which was expecting a decrement
of 1. Changed code to only continue looping if the size of the
Windows array is still greater than 0.

Bug: 18202119
Change-Id: I6124717272c552ec98e89cbacaadcd964fdba02e