This gives a lot more flexibility to the user to cover bandwidth, power,
and privacy issues related to network traffic. The current implementation
does not represent these prefs as well as it should. For example, it does
not force the traffic over the preferred network type if the other type is
set to "never". Instead it just tracks the "unmetered" status of the
active network, and acts based on that.
closes#1381
* This installer is invoked when for non-apk/media files, and
copies them to an appropriate folder on the sdcard.
* We also introduce a FileInstallerActivity to ask for storage
permissions at runtime, as needed by Android 6.0 and above,
and handle the install/uninstall process.
* A toast is shown with the install path after installation.
TODO:
* Manage Installed Apps screen doesn't show media files.
