[plugin] Add a caching mechanism for CoverImage (#7510)

frontend/device/android/device.lua

@@ -392,7 +392,20 @@ function Device:canExecuteScript(file)
 end
 
 function Device:isValidPath(path)
-    return android.isPathInsideSandbox(path)
+    -- the fast check
+    if android.isPathInsideSandbox(path) then
+        return true
+    end
+
+    -- the thorough check
+    local real_ext_storage = FFIUtil.realpath(android.getExternalStoragePath())
+    local real_path = FFIUtil.realpath(path)
+
+    if real_path then
+        return real_path:sub(1, #real_ext_storage) == real_ext_storage
+    else
+        return false
+    end
 end
 
 function Device:showLightDialog()
@@ -432,6 +445,15 @@ function Device:untar(archive, extract_to)
     return android.untar(archive, extract_to)
 end
 
+-- todo: Wouldn't we like an android.deviceIdentifier() method, so we can use better default paths?
+function Device:getDefaultCoverPath()
+    if android.prop.product == "ntx_6sl" then -- Tolino HD4 and other
+        return android.getExternalStoragePath() .. "/suspend_others.jpg"
+    else
+        return android.getExternalStoragePath() .. "/cover.jpg"
+    end
+end
+
 android.LOGI(string.format("Android %s - %s (API %d) - flavor: %s",
     android.prop.version, getCodename(), Device.firmware_rev, android.prop.flavor))