Stores serialized entity data to the extras

libtextclassifier (native side) will serialize the extra entity information
(like parsed datetime) into a byte array and passed it to framework.
Framework puts it to the extras Bundle of result object, i.e. TextClassification
and ConversationActions.

In the future, we will provide a library (maybe AndroidX) to deserialize the
string and return structured objects.

BUG: 129119759

Test: atest frameworks/base/core/tests/coretests/src/android/view/textclassifier/

Change-Id: I8091a1038691419825f5d6da3562b8ba81787dc7