libsnapshot: resume_point_count

We want to add a resume_point_count in the header to represent how many
resume points we've written. In the case that we've written less than
resume_buffer_size, we only want to read the valid resume points.

without these changes incremental OTA runs into segfault or have faulty
data when trying to FindResumeOp() as our resume points contain invalid
entries

Test: full ota followed by inc ota on cuttlefish
Change-Id: I0a8971955439639f2d0f39d9d518c1145ae15c3d