Plugin the easy way - removed all mono_runtime_invoke stuff
AppDelegate may not have active window:

In Unity's case, we have to get the main window a different
way.  However, maybe iOS is now automatically updating the key
window when the current key window is freed, because even
with this commented out, everything seems ok. (e.g. input is
still accepted on the original window).
Make sure the weird FB init is hit during OKManager init:

It is crucial to the function of the SDK, unfortunately.  And an end developer does not
expect to make a second call to OKManager for it