Serialization for OlmAccount and OlmSession OK

...k/src/androidTest/java/org/matrix/olm/OlmAccountTest.java

.../androidTest/java/org/matrix/olm/OlmGroupSessionTest.java

...k/src/androidTest/java/org/matrix/olm/OlmSessionTest.java

...bSdk/olm-sdk/src/main/java/org/matrix/olm/OlmAccount.java

...dk/olm-sdk/src/main/java/org/matrix/olm/OlmException.java

...bSdk/olm-sdk/src/main/java/org/matrix/olm/OlmSession.java

java/android/OlmLibSdk/olm-sdk/src/main/jni/olm_account.cpp

java/android/OlmLibSdk/olm-sdk/src/main/jni/olm_jni.h

...android/OlmLibSdk/olm-sdk/src/main/jni/olm_jni_helper.cpp

java/android/OlmLibSdk/olm-sdk/src/main/jni/olm_session.cpp

java/android/OlmLibSdk/olm-sdk/src/main/jni/olm_session.h