WIP: Replace session states in the Engine classes

...st/java/org/briarproject/IntroductionIntegrationTest.java

.../briarproject/api/introduction/IntroductionConstants.java

...e/src/org/briarproject/introduction/IntroduceeEngine.java

.../src/org/briarproject/introduction/IntroduceeManager.java

...org/briarproject/introduction/IntroduceeSessionState.java

...e/src/org/briarproject/introduction/IntroducerEngine.java

.../src/org/briarproject/introduction/IntroducerManager.java

...org/briarproject/introduction/IntroducerSessionState.java

...rg/briarproject/introduction/IntroductionManagerImpl.java

.../src/org/briarproject/introduction/IntroductionState.java

.../org/briarproject/introduction/IntroduceeManagerTest.java

.../org/briarproject/introduction/IntroducerManagerTest.java

...riarproject/introduction/IntroductionManagerImplTest.java