Make Editor.SuggestionsPopupWindow private again

Editor.SuggestionsPopupWindow was originally introduced as a private
class [1], then we made it public [2] so that a test class can access
it, but that test was later updated [3] and now doesn't have any
direct dependency on Editor.SuggestionsPopupWindow.

In short, we can make Editor.SuggestionsPopupWindow a private class
again without losing anything.  This actually addresses the following
warning from doclava.

   Public class android.widget.Editor.SuggestionsPopupWindow
   extends private class android.widget.Editor.PinnedPopupWindow

There should be no behavior change.

 [1]: If863107681ce82a1639f21315878f830c2991fb7
      6934044f
 [2]: Idf166cbecb3e33be213e4104cf1afd827906f2ad
      a60160b3
 [3]: I12e522436a83fa264cd22176d054877eec411708
      31eb74fb

Fix: 117519592
Test: No doclava warning regarding SuggestionsPopupWindow during full build
Change-Id: Ie722156d3819d05d3cd479527abb8b4750b332f7