Merge pull request #6599 from scottdixon/uiautomator2

Docs: UIAutomator 2

docs/en/writing-running-appium/uiautomator2.md

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+# UIAutomator 2
+
+While the API remains almost the same, the internal implementation has changed and we see the introduction of UIObject2
+
+### UIObject2
+
+Unlike UiObject, the UIElement is bound to a particular view instance and can become stale if the underlying view object is destroyed. As a result, it may be necessary to call findObject(BySelector) to obtain a new UiObject2 instance if the UI changes significantly.
+
+### The Build System
+
+UIAutomator 2 utilizes capabilities of Gradle as a build system. Previous, UIAutomator used Maven/Ant. 
+
+### Test Assets
+
+Test packages produced are now APKs. Previously, UIAutomator produced .jar or .zip files. This makes UIAutomator 2 completely Android instrumentation capable.
+
+### ADB
+
+ADB deals with UIAutomator 2 slightly differently.
+
+The original version of UiAutomator ran as a shell program:
+
+```adb shell uiautomator runtest ...```
+
+UiAutomator 2 is based on Android Instrumentation. Tests are compiled to APKs, and run in an application process:
+
+```adb shell am instrument ...```