healthd: Convert multiple private methods into static nonmember functions

The private methods in class BatteryMonitor do not use the 'this'
pointer. Hence convert these into static nonmember functions.

Test: Compile-tested only.
Change-Id: I4b9b76c9697f1db894ba9fb939ce096ee6b7ff3b
Signed-off-by: Bart Van Assche <bvanassche@google.com>