[tizen][mediacontroller] Change toJSON implementation to simplify serialization.

[tizen][mediacontroller] Change toJSON implementation to simplify serialization.

Before this commit, in order to send Bundle object to the C++ layer
it was necessary to call Bundle.toString() method, and than picojson::parse()
in C++ to deserialize it. This commit removes the need for both this steps.
They will be done implicitly by the framework.

[Verification] bundle js tests 100% pass rate.

Signed-off-by: Michal Michalski <m.michalski2@partner.samsung.com>
Change-Id: I89b11bc634b56b720dd8a5ecea8537ef1ce51b24