provide option to set packagename to load shared preference file

Developer can use private data file from other apps, if it is signed by same certs and specifies same sharedUserId in the manifest. Default token storage writes into SharedPrefs in private mode using package from given context. It is possible to load another context that app has permission and access private file. 

Developer can set packagename to target at AuthenticationSettings. DefaultTokenCacheStore will check that setting to allow shared file loading.

SharedPreferencesImpl handles syncronized get/put requests.