healthd: Fixups for systems without batteries or removable batteries

* Replace unnecessary warnings about missing attributes with a more
  informative message when no battery devices provided by the system.

* Turn off periodic battery checks when no battery devices (thereby
  reducing unnecessary kernel log spam).

* Replace battery properties in log messages with a more informative
  message when no battery is provided or the battery is removed.

Change-Id: I68a514aa7315ae2b5d22cb8861d3c9b1b38035a1