Better API for low disk space warnings.

Provides uniform interface to ask about low disk thresholds; can
be mocked by other tests.  Opens door to adjust thresholds based on
disk type.

Switch monitor service to using new API, and use filesystem paths
from Environment instead of hard-coding.

Change-Id: Ifdb536e36a453f1b67bc65849037ec3cc0232cf2
3 files changed