Screensaver: add dynamic battery symbol to sleep screen message (#12548)

- %B was added to frontend/ui/screensaver.lua to reference the battery symbol
  generated by powerd
- The default English screensaver help message was updated informing users
  of the addition

frontend/ui/screensaver.lua

@@ -116,6 +116,7 @@ function Screensaver:expandSpecial(message, fallback)
     -- %h time left in chapter
     -- %H time left in document (if there are hidden flows, time left in current flow)
     -- %b battery level
+    -- %B battery symbol
 
     if G_reader_settings:hasNot("lastfile") then
         return fallback
@@ -133,6 +134,7 @@ function Screensaver:expandSpecial(message, fallback)
     local time_left_chapter = _("N/A")
     local time_left_document = _("N/A")
     local batt_lvl = _("N/A")
+    local batt_symbol = _("N/A")
     local props
 
     local ReaderUI = require("apps/reader/readerui")
@@ -192,8 +194,10 @@ function Screensaver:expandSpecial(message, fallback)
         local powerd = Device:getPowerDevice()
         if Device:hasAuxBattery() and powerd:isAuxBatteryConnected() then
             batt_lvl = powerd:getCapacity() + powerd:getAuxCapacity()
+            batt_symbol = powerd:getBatterySymbol(powerd:isAuxCharged(), powerd:isAuxCharging(), batt_lvl / 2)
         else
             batt_lvl = powerd:getCapacity()
+            batt_symbol = powerd:getBatterySymbol(powerd:isCharged(), powerd:isCharging(), batt_lvl)
         end
     end
 
@@ -207,6 +211,7 @@ function Screensaver:expandSpecial(message, fallback)
         ["%h"] = time_left_chapter,
         ["%H"] = time_left_document,
         ["%b"] = batt_lvl,
+        ["%B"] = batt_symbol,
     }
     ret = ret:gsub("(%%%a)", replace)
 
@@ -329,7 +334,8 @@ Enter a custom message to be displayed on the sleep screen. The following escape
   %p percentage read
   %h time left in chapter
   %H time left in document
-  %b battery level]]),
+  %b battery level
+  %B battery symbol]]),
         input = screensaver_message,
         buttons = {
             {