healdth: A capacity value of 0 is valid

http://jira.omnirom.org/browse/OMNI-676

During booting when this is checked it is possible that
the kernel drivers hasnt set the real capacity and will
return 0 but still the path is valid

Change-Id: Ie6469f8209225c1079da0db21e3f037eb9bececc
Merge remote-tracking branch 'omni/android-4.4' into defy-4.4