storaged: use sp<> to keep refcount for storaged_t object

BatteryListener takes sp<> as parameter. Passing raw storaged_t ptr
to BatteryListener will cause the ptr to be freed when BatteryListener
releases the sp<>. Keep a refcount in storaged to prevent this from
happening.

Test: kill healthd while storaged is running
Bug: 36652060
Change-Id: I96bc45a3bedb39eb7158b8f6c86334b5b31c9346