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
13 files changed