[Java] propagate generics from outer unmodifiable/synchronized

[chaokunyang]

Is your feature request related to a problem? Please describe.
In #188 and #191, we optimize serialization perf by a special serializer, but that serializer doesn't propagate generics to inner collection element. The serialization will be inefficent in space and perf.

Describe the solution you'd like
Update depth directly so inner collection/map can just use outer collection/map generics.

Describe alternatives you've considered
Get generics first then push it into Generics

Additional context
#188 #190