add some commands for Android and class chain for iOS

[KazuCocoa]

Android

• /wd/hub/session/{sessionId}/appium/device/system_bars
• /wd/hub/session/{sessionId}/appium/device/display_density

iOS

• class chain : https://github.com/facebook/WebDriverAgent/wiki/Queries
  • https://github.com/appium/appium-xcuitest-driver/blob/325d38e0a7875fce80a4b0cb106d5dd8a624991f/lib/commands/find.js#L23