Add GN targets for AppRTC Demo on Android.

Adds GN equivalents for following targets:
AppRTCDemo          -> //webrtc/examples:AppRTCDemo
AppRTCDemo_apk      -> //webrtc/examples:AppRTCDemo_lib (kind of)
AppRTCDemoJUnitTest -> //webrtc/examples:AppRTCDemoJUnitTest
AppRTCDemoTest      -> //webrtc/examples:AppRTCDemoTest
libjingle_peerconnection_java -> //webrtc/api/libjingle_peerconnection_java
libjingle_peerconnection_so   -> //webrtc/api/libjingle_peerconnection_so

New GN targets:
//webrtc/base:base_java
//webrtc/examples:AppRTCDemo_resources
//webrtc/examples/androidapp/third_party/autobanh:autobanh_java

BUG=webrtc:6035
R=magjed@webrtc.org, tommi@webrtc.org

Review URL: https://codereview.webrtc.org/2105803002 .

Cr-Original-Commit-Position: refs/heads/master@{#13322}
Cr-Mirrored-From: https://chromium.googlesource.com/external/webrtc
Cr-Mirrored-Commit: 9c0c75bd6edf80d7a2edd6028ac6b96393d063a7