Keep instance of BinderInputWindow for the same token.

Some code in InputDispatcher uses pointer value comparison to track if a
BinderInputWindow is the same. Therefore just keep the initial instance
and reuse that for every update.

This is better than changing all pointer comparisons to token
comparisons because it would be hard to make sure every comparison is
changed.

Later we could simply drop InputWindowHandle and use InputWindowInfo
value instead. It would also be better if it could implement move
constructor so that we can use move semantics.

Bug: 120289807
Bug: 120463595
Bug: 120481017
Test: Verified that input focus stops constantly changing by inpecting
logcat after enabling DEBUG_FOCUS in InputDispatcher.cpp. Freeform
drag-resize works with a workaround to another race condition.

Change-Id: I34bbf8c076373ef23494f858536c5188eff95abd