Android: Use Back to open the emulation menu on all devices

https://bugs.dolphin-emu.org/issues/12029

We currently have one way of opening the menu on touch screen
devices (swiping down from the top of the screen to bring up the
action bar and selecting the menu in the action bar), and another
way of opening the menu on Android TV (pressing Back). However,
some devices that claim to support touch (or don't support
leanback? Dolphin currently conflates the two) don't actually let
you swipe down from the top of the screen in the way that Dolphin
expects, notably Chromebooks. There are also some phones where you
can swipe down from the top of the screen but this for some reason
doesn't lead to the action bar becoming visible, though we are
getting less reports about this nowadays than in the past.

This change makes us use the Back method on all devices,
since it should work on all devices with no significant drawbacks.

Unfortunately, we not only have two different ways of triggering
the menu but actually two entirely different menus, with the
non-touch menu not implementing options that only are revelant
when using a touch screen. A later commit will add the missing
features to the menu that we now use on all devices.

Source/Android/app/src/main/res/values/strings.xml

@@ -355,7 +355,7 @@
     <string name="emulation_control_joystick_rel_center">Relative Stick Center</string>
     <string name="emulation_control_rumble">Rumble</string>
     <string name="emulation_choose_controller">Choose Controller</string>
-    <string name="emulation_touch_button_help">Swipe down from the top of the screen to access the menu.</string>
+    <string name="emulation_menu_help">Press Back to access the menu.</string>
     <string name="emulation_touch_overlay_reset">Reset Overlay</string>
     <string name="emulation_ir_group">Touch IR Pointer</string>
     <string name="emulation_ir_sensitivity">IR Sensitivity</string>