feat: Communication client tests

[elefantel]

Fixes #131

• Add unit tests for CommunicationClient - the client responsible for AIDL communication with MetaMask wallet
• Add Mockito testing dependency for mocking application context and for stubbing behaviour of bound service connections
• Add CommunicationClientModule - a factory class for producing the client's dependencies
• Encapsulate the AIDL bound service to improve modularity and testability by moving it out from CommunicationClient into
  • ClientServiceConnection for managing connections and sending messages
  • ClientMessageServiceCallback for managing received messages

[BjornGunnarsson]

@andreahaku can you take a look at this

[andreahaku]

small comment. the rest is ok.

[andreahaku]

LGTM