Provide content Uris to downloaded apks via FileProvider
* moves apk verification back inside the Installer class
* uses support libs FileProvider for content Uris
* move apk file caching and storage methods into ApkFileProvider class
Some of the ugly version checks for Android N can be removed after Android N has been released. Unfortunately Google decided to keep SDK version at 23 for Android N dev preview and only change the CODENAME, thus ``Build.VERSION.SDK_INT <= Build.VERSION_CODES.M`` returns true on Android N preview :/ , see https://commonsware.com/blog/2016/03/17/backwards-compatibility-n-developer-preview.html
Tested on Android N dev preview 3 emulator, Android 6 stock and Android 5.1 rooted with priv extension.
See merge request !331
34aa8ab062