Clean up tests
- Broke up ConstantsTest (#280 (closed)) - the key encoding parts are now in KeyEncodingAndParsingTest, the message encoding parts are in MessageSizeIntegrationTest
- Renamed the other integration tests in briar-android-tests
- Moved the integration tests in briar-android-tests to the top-level package, as they all involve code from multiple packages
- Separated DatabaseExecutorModule from DatabaseModule so we can use a different @DatabaseExecutor in integration tests
- Merged AppModule with AndroidModule (@ernir, this touches code you're working on but I don't think there are any conflicts)
- Renamed some TestUtils methods for consistency
- Used TestUtils.getRandomBytes() where applicable
Fixes #280 (closed).