rebuild index when underlying OS is upgraded
If Android is upgraded, then some of the assumptions of how we are using the database break. For example, permissions are parsed based on Build.VERSION.SDK_INT and that result is then stored in the database. If upgrading from android-22 to android-23 for example, then any <uses-permission-sdk-23> permissions should take effect, but they will have been filtered out.
So F-Droid should store shared preference that is SDK_INT when F-Droid was installed. Then whenever it starts, it should compare that to the current SDK_INT, and if its newer, rebuild the index.