healthd: make periodic battery status a debug message

Battery status is printed to the kernel log as a warning every
10 seconds.
[  276.605006] healthd: battery l=88 v=4235 t=29.6 h=2 st=2 c=0 chg=u
[  286.662791] healthd: battery l=88 v=4238 t=29.5 h=2 st=2 c=0 chg=u
[  296.707097] healthd: battery l=88 v=4238 t=29.5 h=2 st=2 c=0 chg=u

That's a debug statement and not a warning.

Change-Id: I45cfdeb150de59e2ef68749e0ab400da3599d156