Implement the ChatMessageValidator class

The constructor interface has changed slightly to what was initially
planned in order to make the interface of the class cleaner, as well
as easier to test.
AndroidManifest: add directives to facilitate Android JUnit tests

The directives are necessary in order to allow running JUnit tests
in the Android environment.
Add unit tests for the ChatMessageValidator class

The unit tests make sure that the validator's behavior matches the
expected one -- it gives the same results of validation as the TCA
server would.