qml: remove back button in top bar, move menu button to left

add AppController.isAndroid function to test for android platform. Disable fade in if not on android

electrum/gui/qml/qeapp.py

@@ -188,6 +188,9 @@ class QEAppController(QObject):
         else:
             self._plugins.disable(plugin)
 
+    @pyqtSlot(result=bool)
+    def isAndroid(self):
+        return 'ANDROID_DATA' in os.environ
 
 class ElectrumQmlApplication(QGuiApplication):