cashew: default policy: reduce request timer frequency

Reduce default request timer frequency from once per minute to once
every ten minutes. This will reduce load on the usage API and reduce
user data consumption.

Add unit tests for DefaultPolicy

BUG=chromium-os:8188, chromium-os:7775
TEST=unit tests, manual testing on device

Change-Id: I015e3152971e6dad32cc36e77b96e9e8bf753cb4

Review URL: http://codereview.chromium.org/4201001
3 files changed