System crash fixed when turn off MagnificationGesture

Symptom:
WindowManagerService throws exception and system crashes.

Root cause:
When the MagnificationGesture is turned off during animation of scale
up/down, WindowManagerService.mAccessibilityController can be null so
exception is thrown.

Solution:
Cancel ongoing animation before turning off MagnificationGesture.

Change-Id: I469d84e024bdd453e3729a69229398233ed2eb61