fix os.execute implementation (#524)

Just forward the whole command to `sh -c`, so there's no need
to muck around with escaping, and all KOReader's use-cases are
supported (redirection, multiple commands, etc…).

assets/android.lua

@@ -2725,13 +2725,7 @@ local function run(android_app_state)
 
     os.execute = function(command) -- luacheck: ignore 122
         if command == nil then return -1 end
-        local argv = {}
-        command:gsub("([^ ]+)", function(arg)
-            -- strip quotes around argument, since they are not necessary here
-            arg = arg:gsub('"(.*)"', "%1") -- strip double quotes
-            arg = arg:gsub("'(.*)'", "%1") -- strip single quotes
-            table.insert(argv, arg)
-        end)
+        local argv = {'sh', '-c', command}
         return android.execute(unpack(argv))
     end