Add new "storage not low" job scheduler constraint.

This allows you to say that your job should run only when device
storage is not low.

Adds new command line interface to DeviceStorageMonitor to help
with driving the tests (modelled after BatteryService).

Test: new StorageConstraintTest suite.
Change-Id: I96bfb761cd8257b6f68dde43ce9cfb1a3b9d0acb