[Java] Fixed a bug in Java DB Benchmark where random reads does not consider full key range.

Summary: Fixed a bug in Java DB Benchmark where random reads does not consider full key range.

Test Plan:
make rocksdbjava
make jdb_bench
cd java
jdb_bench.sh --db=/tmp/rocksdb-test --benchmarks=fillseq --use_existing_db=false --num=100000
jdb_bench.sh --db=/tmp/rocksdb-test --benchmarks=readrandom --use_existing_db=true --num=100000 --reads=1000000

Reviewers: haobo, sdong

Reviewed By: haobo

CC: leveldb

Differential Revision: https://reviews.facebook.net/D18693