Give DeviceStorageMonitorService it's own thread.

When devices are low on space, it calls down into installd to clear
cached files, which can block for a long time.  Instead of risking
the wrath of the Watchdog on IoThread, give ourselves a separate
thread.

Test: builds, boots
Bug: 64080821
Change-Id: Iaca00966bb836d58e8d2400d6c9d544023ef1af8