Change Checkpoint API to return total number of checkpoints

Fixes a race condition with SetStateUnsafe that caused some
warnings in the Barrier::~Barrier.

The race was:
RunCheckpoint sees suspended thread, runs the checkpoint. Inside the
checkpoint, the thread state had changed to runnable by
SetStateUnsafe. This occasionally caused more Barrier::Pass than
expected.

The fix is to return the total number of checkpoints instead of just
the runnable ones.

Bug: 24191051
Change-Id: If15a933ed4c8efa66a5f27cd5feaa2e5957ae804