healthd: Only use battery values that seems legit

On some devices, there are other sysfs paths having a "capacity"
node than just the battery, which can return a weirdo value. We
make sure we only use legit battery levels (> 0%).

Change-Id: I3caf34e7fed4b89f953ec98c486b53b8786c8f74
Merge "healthd: Only use battery values that seems legit" into android-4.4