Doze: Use Settings.Global.DEVICE_IDLE_CONSTANTS instead of hardcoded constants

This will allow gservices to tweak the settings and experiment
with different values for various doze constants.

The values are encoded in a string as a key=value list. Ex:

inactive_to=5000,idle_factor=0.01

Bug:21640379
Change-Id: Ie98a0e4893f9b46a64d961d6c5c5169b8b8ad742