* Remove ScratchJrApplication class - it was only being used to initialize the old Google Analytics.
* Change AppUsage (home, school, other, noanswer) from being a prefix on `label` to being a Firebase user property.
* Set the user property when loading the index page - it shouldn’t change for the rest of the session.
PBS Kids has automated testing that looks for uses-sdk with a minimum targetSDK of 23. Even though this gets overridden in the build.gradle settings, adding a line with default values to get through their testing.
Changed android:requiresSmallestWidthDp from 600 to 500.
Android suggests that 600 is a normal value for 7 inch tablets and up. It turns out that many 7-inch tablets are in the 500-600 range (half of the Amazon ones for example).
Fixes#85
