Make window callbacks non static member of ViewRootImpl

Move add/removeWindowCallbacks to onAttachedToWindow/onDetachedFromWindow.

There might be more than one windows in an app process, the window
callbacks need to be per window, not global.

bug: 24679461
Change-Id: I216ff27b2a41ecfe7399a8161df362bebc0ac96a
2 files changed