sync: store the num_fences from first SYNC_IOC_FILE_INFO

Fixes a bug with the signal time of devices using the
modern sync file interface. The bug only affects kernels running
an Android kernel 4.9 and later.

b/63395253

Test: tests/sync_test.cpp

Change-Id: I6fb00bcb8e16a3268c357153edd8e35a44546caa
1 file changed