healthd: Set fixed battery level and temperature via properties

setprop persist.sys.battery.capacity 77
setprop persist.sys.battery.temperature 123

and reboot to cause a fixed battery level of 77% and temperature of 12.3C
to be reported to Android.

Typically used on power evaluation boards without batteries connected.

Bug: 14839868
Change-Id: Ibae5e16429d05891cb0787d74a2fe93b07013699