metricsd: Add more assert clause.

This CL adds two more checks:
* When populating a closed log with the metadata, make sure the
  SystemProfileSetter is not null.
* When using the update engine client library, check if the update
  engine client object was created correctly. The returning object can
  be null if update engine's binder service is not running.

Bug: 26731548

Change-Id: I8a615c87f33e15a163d56ccfa3a932a0908aac5f