Use daemon threads for battery and property background fetches.

The thread pool previously used had no benefit over a simple thread,
since there was a thread pool per device.

Using daemon threads so having an active prop or battery query does not
prevent JVM from shutting down.

Also remove the battery fetch backoff time to be consistent with
property fetch.

Change-Id: Ica14c161462f7a5ee1729d87228f4d13fb28de63
5 files changed