Add getExperimentFlagInt to utility

A utility used for getting the integer from the experiment flag.
No functionality change.

Bug: 120182528
Bug: 141218721
Test: manually setting the device_config works, e.g.
      adb shell device_config put netd_native dot_connect_timeout_ms 3000
Change-Id: Ib4bb39b0465de86c3db0151f0b3c3a66f5a83de8