The latest Android SDK API 23 made some changes impacting packaging or launch on for Unreal projects. The AndroidWorks installer included in Engine/Extras/Android should only install up to API 22 by default but if you are using 1R2 from the NVIDIA website it may be pulling down 23 as well.
API 23 removed org.org.apache.http from the standard package and moved it to optional. Unfortunately, it also removed some depreciated API calls which Google’s APK expansion downloader depends on (we use it for OBB download). The best thing to do for now is use no later than SDK API 22.
The Android SDK section of Project Settings allows setting the SDK API Level and NDK API Level. By default, SDK API Level is “latest” and NDK API Level is “android-19”. The NDK API Level should be set to “android-19” if you wish to target devices before Lollipop (5.0). If you set this to “android-21” or higher, your APK will only work on 5.0 devices.
SDK API Level set to “latest” will use the highest number found installed. If you have API-23 installed, you should change the SDK API Level from “latest” to “matchndk” or “android-21” (check your SDK’s platforms directory for which versions are available). It is OK for SDK to be set higher than NDK.