With the original logic, if an app creates a system window, when the
user goes to home screen, the system window will be still there and
become unable to receive input events, because the system window will
be also changed to the stopped state with the app window, and the
current logic of ViewRootImpl forbid a stopped window receiving input
events.
This change prevents assigning the token of the app window to system
windows created by the app, so that when the app goes to the stopped
state, its system windows won't be affected (can still receive input
events).
This change is related to the following changes:
a5d29971f815ed2754a3c3672cd3f741725dedc3
Bug: 24967303
Bug: https://code.google.com/p/android/issues/detail?id=189710
Change-Id: I515e47bafcf39a2b1bdf92f11f623bef8fb6263c
20ebb4c2d4