Set default ClassLoader for Parcel readSerializable API.

Set the default ClassLoader for the readSerializable(ClassLoader,
Class) API, when the ClassLoader parameter is null.

Doing so could enhance the security of Parcel deserialization,
as it would prevent resolving the Serializable class using unexpected
ClassLoaders.

Test: atest -d android.os.cts.ParcelTest
Bug: 195622897
Change-Id: I6da4b4f817c33e4464d90d1e9775b54793835c92