Automatically load ActivityManager DeviceConfig constants.

The former method required us to load each individual DeviceConfig
constant at startup time. This could easily lead to an issue where
someone forgets to add that line in the startup path and the value
isn't loaded. Switching to have all DeviceConfig constants read and
processed by a generic loop so we don't run into this issue.

Bug: N/A
Test: flash device and confirm DeviceConfig constants are loaded
Change-Id: I981829722cf035e894415a11e214724b213c65fa