Update anroidapp/README for GN build.

webrtc/examples/androidapp/README

 This directory contains an example Android client for https://appr.tc
 
 Prerequisites:
-- "Getting the code" on http://www.webrtc.org/native-code/android
-- Set up webrtc-related GYP variables:
-  export GYP_DEFINES="OS=android $GYP_DEFINES"
-- When targeting both desktop & android, make sure to use a different output_dir
-  value in $GYP_GENERATOR_FLAGS - for example
-  export GYP_GENERATOR_FLAGS="$GYP_GENERATOR_FLAGS output_dir=out_android"
-  or you'll likely end up with mismatched ARM & x86 output artifacts.
-  If you use an output_dir other than out/ make sure to modify the command-lines
-  below appropriately.
-- Finally, run "gclient runhooks" to generate Android-targeting .ninja files.
+- "Getting the code", "Compiling", and "Using the Bundled Android SDK/NDK"
+  on http://www.webrtc.org/native-code/android
 
 Example of building & using the app:
 
 cd <path/to/webrtc>/src
-ninja -C out/Debug AppRTCDemo
-adb install -r out/Debug/apks/AppRTCDemo.apk
+ninja -C out/Default AppRTCDemo
+adb install -r out/Default/apks/AppRTCDemo.apk
 
 In desktop chrome, navigate to https://appr.tc and note the r=<NNN> room
 this redirects to or navigate directly to https://appr.tc/r/<NNN> with
 your own room number. Launch AppRTC on the device and add same <NNN> into the room name list.
 
 You can also run application from a command line to connect to the first room in a list:
 adb shell am start -n org.appspot.apprtc/.ConnectActivity -a android.intent.action.VIEW
 This should result in the app launching on Android and connecting to the 3-dot-apprtc
 page displayed in the desktop browser.
 To run loopback test execute following command:
 adb shell am start -n org.appspot.apprtc/.ConnectActivity -a android.intent.action.VIEW --ez "org.appspot.apprtc.LOOPBACK" true