[Core] Fix AWSKeyValueStore for stringSet operations and propagate exceptions when encryption key cannot be loaded/created

Author: Karthikeyan Vasuki Balasubramaniam

aws-android-sdk-core/src/main/java/com/amazonaws/internal/keyvaluestore/AWSKeyValueStore.java

@@ -19,7 +19,6 @@
 import android.content.SharedPreferences;
 import android.os.Build;
 
-import com.amazonaws.auth.policy.actions.SecurityTokenServiceActions;
 import com.amazonaws.logging.Log;
 import com.amazonaws.logging.LogFactory;
 import com.amazonaws.util.Base64;