Adapt to google::protobuf::int64 type change

Protobuf 3.9.1 redefines google::protobuf::int64 from long long
to int64_t, which is sometimes long and sometimes long long.  Use
PRIi64 to print it.  Also temporarily cast to int64_t to decouple
this change from the change that updates protobuf.

Bug: 117607748
Test: m checkbuild
Change-Id: I482b957d2262e5001140ed1153c7de2b17facae2