Convert Hermesdebugger to device plugin

Summary:
This diff exposes the Hermes debugger as a device plugin, for similar reasons as we did with React Devtools.

Also replaced an ugly hack with another just as ugly hack (rendering outside the React tree to preserve the state of the chrome devtools when switching to another plugin and coming back), that behaves at least correctly when resizing etc instead of doing absolute styling

Reviewed By: passy

Differential Revision: D19905082

fbshipit-source-id: 1e80c55fc9c5b1f8a9292f0ba1ef66b0b007b7fc

src/App.tsx

@@ -148,6 +148,10 @@ export class App extends React.Component<Props> {
           ) : (
             <PluginContainer logger={this.props.logger} />
           )}
+          <div
+            id="flipper-out-of-contents-container"
+            style={{width: '100%', display: 'none'}}
+          />
         </FlexRow>
         <StatusBar />
       </FlexColumn>