[REFACTOR] health 2.0: BatteryMonitor::update split into 3 funcs

BatteryMonitor::update no longer calls healthd_board_battery_update
and callback for us.

Test: VTS health HAL 2.0 test
Change-Id: I05fbf60b7e2c38f82e019d2fec2b5f535defaeae
1 file changed