Register callback to statsD for AppSearchStorageInfo

Bug: b/173532925, b/193254235
Test: statsd_testdrive 10116
      atom {
        app_search_storage_info {
          user: 0
          total_storage_size_bytes: 3929088
          document_storage_info {
            num_alive_documents: 1116
            num_deleted_documents: 4933
            num_expired_documents: 0
            document_store_size_bytes: 3637760
            document_log_size_bytes: 2928640
            key_mapper_size_bytes: 49152
            document_id_mapper_size_bytes: 49152
            score_cache_size_bytes: 122880
            filter_cache_size_bytes: 73728
            corpus_mapper_size_bytes: 20480
            corpus_score_cache_size_bytes: 4096
            namespace_id_mapper_size_bytes: 20480
            num_namespaces: 21
          }
          schema_store_storage_info {
            schema_store_size_bytes: 39424
            num_schema_types: 4
            num_total_sections: 9
            num_schema_types_sections_exhausted: 0
          }
          index_storage_info {
            index_size_bytes: 248320
            lite_index_lexicon_size_bytes: 155648
            lite_index_hit_buffer_size_bytes: 262144
            main_index_lexicon_size_bytes: 0
            main_index_storage_size_bytes: 0
            main_index_block_size_bytes: 4096
            num_blocks: 1
            min_free_fraction: 0.99999905
          }
        }
      }
      appsearch cts tests

Change-Id: I009c0932e36afb467c93013215d1e67dfae5a28a