android: add a toggle to ignore back button events (#6269)

Requires koreader/android-luajit-launcher#233
Requires koreader/koreader-base#1117

It is a workaround for #6263 but can be useful on some devices with erratic back key behaviour too (yep, Onyx)

frontend/device/android/device.lua

@@ -224,6 +224,11 @@ function Device:init()
         android.setVolumeKeysIgnored(true)
     end
 
+    -- check if we ignore the back button completely
+    if G_reader_settings:isTrue("android_ignore_back_button") then
+        android.setBackButtonIgnored(true)
+    end
+
     -- check if we enable a custom light level for this activity
     local last_value = G_reader_settings:readSetting("fl_last_level")
     if type(last_value) == "number" and last_value >= 0 then

frontend/ui/elements/common_settings_menu_table.lua

@@ -239,6 +239,16 @@ if Device:isAndroid() then
         callback = function() G_reader_settings:flipNilOrFalse("camera_key_toggles_touchscreen") end,
     }
 
+    common_settings.android_back_button = {
+        text = _("Ignore back button completely"),
+        checked_func = function() return android.isBackButtonIgnored() end,
+        callback = function()
+            local is_ignored = android.isBackButtonIgnored()
+            android.setBackButtonIgnored(not is_ignored)
+            G_reader_settings:saveSetting("android_ignore_back_button", not is_ignored)
+        end,
+    }
+
     -- fullscreen toggle on devices with compatible fullscreen methods (apis 14-18)
     if Device.firmware_rev < 19 then
         common_settings.fullscreen = {

frontend/ui/elements/filemanager_menu_order.lua

@@ -55,6 +55,7 @@ local order = {
         "android_volume_keys",
         "android_camera_key",
         "android_haptic_feedback",
+        "android_back_button",
         "----------------------------",
         "invert_page_turn_buttons",
     },

frontend/ui/elements/reader_menu_order.lua

@@ -75,6 +75,7 @@ local order = {
         "android_volume_keys",
         "android_camera_key",
         "android_haptic_feedback",
+        "android_back_button",
         "----------------------------",
         "invert_page_turn_gestures",
         "invert_page_turn_buttons",