adbd: make a PR_CAPBSET_DROP error fatal.

Back in 080427e4, we said:

  If the kernel doesn't support file capabilities, we ignore
  a prctl(PR_CAPBSET_DROP) failure. In a future CL, this could
  become a fatal error.

Make it a fatal error. With SELinux support, all devices are
required to support file capabilities.

Change-Id: I0ce0c4cb9387c42e325cbf1a63c5d82e6aab530c