Added support for single app mode

Added code to support receiving AppsRequest messages and process
them.
AppsRequestType "REFRESH" will take a list of existing apps that
the client has, compare them to currently installed apps on the VM,
and send back an AppsResponse instructing the client which apps to
add, update, or remove. Uses client's screen density to determine
what size icons to send.
AppsRequestType "LAUNCH" will simply launch a given app package.
Also changed Utility.drawableToBytes method to use PNG format
instead of WEBP, to allow support for transparency.

Co-commits:
svmp-android-client
svmp-protocol-def
svmp-server