Clean up tests

* Broke up ConstantsTest (#280) - 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.

See merge request !133