Don't ANR when no windows are touchable.

If the user touches a portion of the screen in which all of the windows
have FLAG_NOT_TOUCHABLE set, handle this as a failed event injection,
so that it doesn't result in ANR.

Change-Id: I6437e3dd1641005b8ac80f6cf7d20b14b97fcc7b
Merge "Don't ANR when no windows are touchable." into jb43