[HostStubGen] Introduce sharding for class processing

Allow hoststubgen and ravenizer to process jar files with multiple
threads to reduce the total processing time.

Bug: 397498134
Flag: EXEMPT host side change only
Test: f/b/r/scripts/run-ravenwood-tests.sh
Change-Id: I228a1ec69fab49eb855246151899b700c0df972c