Removed use of WindowState.mTargetAppToken

Instead of keeping track of the ime target app token in each ime window state
for the purpose of adjusting the animation layer, we now just check if it is
a ime window and there is an ime target when we want to adjust the animation
layer.

Also correct some use of WindowTokens.windows to AppWidowToken.allAppWindows
in AppWindowToken since that contains all the windows associated with the app
token.

Bug: 30060889
Change-Id: Iba1a6076d0ed09413a0802d02c92b9b32ad50891