Reset readsEnabled flag after successfully flushing the primary region

Found the problem when implementing HBASE-21671, TestRegionReplicaFailover is easy to fail. In this test we set REGION_REPLICA_WAIT_FOR_PRIMARY_FLUSH_CONF_KEY to true, which means the secondary replicas can not be read until the primary region has been successfully flushed. But in RegionReplicaFlushHandler, there is only one place where we call region.setReadsEnabled(true), this is incorrect.