WM: Blame the window from the focus request for ANR

If there are no focusable windows, then try to blame the window that
was requested to become focused. This will avoid blaming the wrong
process when WM requests focus on a window that is placed on top
of the focused app and the window fails to become focusable.

Test: atest android.server.wm.AnrTests
Test: steps from bug
Bug: b/23990703
Change-Id: Ibf74086de259a671c77fcee9746048a3a440bcc4