kivy: window.request_focus_for_main_view: fix compat with newer pyjnius

```
Traceback (most recent call last):
  File "kivy/_clock.pyx", line 645, in kivy._clock.CyClockBase._process_events
  File "kivy/_clock.pyx", line 218, in kivy._clock.ClockEvent.tick
  File "/home/user/wspace/electrum/.buildozer_kivy/android/app/electrum/gui/kivy/uix/dialogs/password_dialog.py", line 227, in <lambda>
  File "/home/user/wspace/electrum/.buildozer_kivy/android/app/electrum/gui/kivy/uix/dialogs/password_dialog.py", line 333, in <lambda>
  File "/home/user/wspace/electrum/.buildozer_kivy/android/app/electrum/gui/kivy/main_window.py", line 721, in on_open_wallet
  File "/home/user/wspace/electrum/.buildozer_kivy/android/app/electrum/gui/kivy/main_window.py", line 687, in on_wizard_success
  File "/home/user/wspace/electrum/.buildozer_kivy/android/app/electrum/util.py", line 445, in <lambda>
  File "/home/user/wspace/electrum/.buildozer_kivy/android/app/electrum/util.py", line 441, in do_profile
  File "/home/user/wspace/electrum/.buildozer_kivy/android/app/electrum/gui/kivy/main_window.py", line 940, in load_wallet
  File "/home/user/wspace/electrum/.buildozer_kivy/android/app/electrum/gui/kivy/main_window.py", line 950, in request_focus_for_main_view
  File "jnius/jnius_export_class.pxi", line 844, in jnius.jnius.JavaMethod.__call__
jnius.jnius.JavaException: Cannot call instance method b'requestFocusForMainView' on class b'org/kivy/android/PythonActivity'
```

electrum/gui/kivy/main_window.py

@@ -947,7 +947,8 @@ class ElectrumWindow(App, Logger, EventListener):
         # see #6276 (specifically "method 2" and "method 3")
         from jnius import autoclass
         PythonActivity = autoclass('org.kivy.android.PythonActivity')
-        PythonActivity.requestFocusForMainView()
+        activity = PythonActivity.mActivity
+        activity.requestFocusForMainView()
 
     def update_status(self, *dt):
         if not self.wallet: