FocusRequest token can be nullable

When a freeform app is focused but its window is not ready, the input
focus on the window of the previously focused app should be removed.
To do so, a FocusRequest needs to be send to the InputFlinger, and there
will be no token because the new window is not ready. Change token to be
nullable, so InputRequest can be pass to FocusResolver.

Bug: 216852742
Test: atest AnrTests#slowOnCreateWithKeyEventTriggersAnr
Change-Id: If2ed666a17576557a602a8e46494d8c66d8ce89f