Java fixes for 22.x (#12035)

* Fix mutability bug in Java proto lite: sub-messages inside of oneofs were not
being set as immutable. This would allow code to get a builder for a
sub-message and modify the original (supposedly immutable) copy.

PiperOrigin-RevId: 511598810

* Add casts to make protobuf compatible with Java 1.8 runtime.
Fix for: https://github.com/protocolbuffers/protobuf/issues/11393



PiperOrigin-RevId: 511807920

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Co-authored-by: Protobuf Team Bot <protobuf-github-bot@google.com>