BatteryService: scheduleUpdate asynchronously

IBatteryPropertiesRegistrar.scheduleUpdate calls IHealth.update()
asynchronously.

BatteryStatsImpl calls scheduleUpdate while holding a lock, which
may lead to deadlocks if the remote process calls back to the
framework.

Fixes: 139503418
Test: monitor HealthScheduleUpdate trace points
Change-Id: I8168d7c4e4a0b1d31343360d2c7f6d36c7aa692a