Settings crash after enabling TalkBack accessibility.

1. AccessibilityInput filter was not checking whether the touch
   explorer instance is not null before passing it an accessibility
   event. If the accessibility event is dispatched before the input
   filter is installed but after it is created we runt into this
   case.

2. Added a missing null check in accessibility node info.

bug:6635089

Change-Id: Ia389dc1f427427eb73794f6331ccb870e0b44c55