Fix accessibility drawing

 Bug: 6407623
 If script injection is disabled, the accessibility injector works
 by modifying the text selection. However, this would cause WebView
 to go into text selection mode, showing the CAB and such, which
 we don't want. Add a flag saying WHY text selection is being changed
 so that we can respond accordingly in WebViewClassic.

Change-Id: Ia509def3fcdb022b93fbbc7ed89bc9558663afd3