android: Implement update checker functionality mirroring the Qt frontend (#2265)

* update_checker.cpp: Tiny correctness adjustment

* android: Initial UpdateChecker object implementation

No actual frontend functionality yet

* android: Very initial implementation of frontend update checker notifications

* android: Fix UpdateChecker returnning incorrectly formatted tag when using stable channel

* android: Add downgrade mitigation to update checker, mirroring Qt

* Display new version number in Android update check notification

* android: Fix update checker notification not having a cancel button

* android: Add user-configurable options to control update checker behaviour

* android: Auto-disable update checker for debug builds

* Don't show update checker settings when using Google Play build

Not like they would do anything anyway, but they shouldn't be visible

* android: Improve hardiness of tag decoding in update checker

CMakeModules/GenerateSettingKeys.cmake

@@ -149,6 +149,8 @@ foreach(KEY IN ITEMS
     "web_api_url"
     "citra_username"
     "citra_token"
+    "check_for_update_on_start"
+    "update_check_channel"
 )
     set(SETTING_KEY_LIST "${SETTING_KEY_LIST}\n\"${KEY}\",")
     set(SETTING_KEY_DEFINITIONS "${SETTING_KEY_DEFINITIONS}\nDEFINE_KEY(${KEY})")
@@ -234,8 +236,6 @@ if (ENABLE_QT)
         "pauseWhenInBackground"
         "muteWhenInBackground"
         "hideInactiveMouse"
-        "check_for_update_on_start"
-        "update_check_channel"
         "inserted_cartridge"
         "enable_discord_presence"
         "iconSize"