libstagefright_foundation: fix printf format for 64-bit

Fixes build error:
frameworks/av/media/libstagefright/foundation/ANetworkSession.cpp:582:131: error: format '%d' expects argument of type 'int', but argument 4 has type 'size_t {aka long unsigned int}' [-Werror=format=]
                 ALOGI("%d datagrams remain queued.", mOutFragments.size());
                                                                                                                                   ^
cc1plus: all warnings being treated as errors

Change-Id: I69d1f086b8f1e450f107671c8f8f3ed696d170b4