Upgrade to Android 4.4

[eoinkelly]

Android 4.4 provides lots of great new stuff for us but the upgrade is not trivial

These are the steps we need to take

• Upgrade grit-android repo to target API level 19 (aka Android 4.4)
• Check that the cordova plugins we use will continue to work
• Upgrade cordova
• Replace images of khmer text with real khmer text in the app
• Remove the generated images and edit the JSON generator script in grit-content

I'm going to do this work in the kitkat branch of all three repositories. We can merge into master when everything is ready.